In its simplest understanding this might mean sniping an expansion or targeting a vulnerable unit, such as mutas on siege tanks. Is the mineral line vulnerable? Drop some hellions. No detection? DTs. In each situation you scout and exploit what your opponent lacks.
But consider this situation: In TvT Terran has a hard contain and has just moved their tanks close enough to assault the opposing main, within firing distance of two targets: the main mineral line and the production facilities. Essentially the attacker has a decision to make. Shut down the economy or army production? Which target should be prioritized?
Without doubt the tanks should attack the production facilities. If one Terran has a hard contain they have an army advantage. The income will probably be exactly alike. Less production buildings reduces the army directly, furthering the likelihood of the Terran ever breaking free. Attacking the mineral line is a less direct way of achieving the same goal.
Here’s another: Your opponent made a risky fast expand and you scout it just as it completes. Do you: fast expand yourself, mass a quick army and push or tech up?
You tech.
Think of Starcraft mechanics as three pillars supporting one edifice: army, tech and economy. The winner is determined by gaining the greatest advantage in one, not all, of those 3 categories, since each relies on the other two to prosper. Army means nothing without an economy to replenish it. Economy means nothing without army because it cannot be defended. Tech will trump army and economy, but good luck completing a tech build without having either.
A fast expansion sacrifices tech. So the best Terran response to a Protoss FE is cloaked Banshee. The best Zerg response to a Terran FE is a fast spire. The best Protoss response to a high econ Zerg is to move quickly to tier 3.
If the player fast expanded in response they would be mirroring their opponent, only less efficiently, since their expansion would be late. In terms of the three pillars, the earlier expansion would yield a higher economy and lesser tech while the later would be more middling, which is a loss since the goal is to create an advantage in one of the three areas.
The reason why it’s important to create an advantage rather than minimize a disadvantage is because in Starcraft (and all strategy games, really) advantages increase exponentially. It’s not like running a race, where moving faster is equally good at every stage in the race. It’s like running a race where the leader moves faster simply because they’re in the lead. Remember, the hardest SCV to build is the first.
So, regarding the FE response, why is it wrong to push with low-tech units?
Because fast expansions sacrifice tech more than army. A one-base Zerg will upgrade to a Lair much faster than a 2-base Zerg, but both will have similar ling counts.
But wait, doesn’t 2-rax work well against a 14-hatch?
Sure, it may result in some successful harassment. But rarely, especially nowadays, does it win the game, and mostly it just evens the playing field by reducing the Zerg economic advantage for expanding early.
That’s why Terrans are switching to Hellion builds in response to Zerg FE’s. Hellions cost higher tech, but the advantage is quickly felt when facing only lings. 2 rax pressure might not win many TvZs, but fast hellions will.
But let’s say the Terran opts instead to respond with 2-port cloaked Banshees, and they get to the Zerg base with enough time to either: kill 20 drones, kill 2 Queens, the Spine Crawler and half the standing army (say 16 lings and 4 banelings), kill the Lair, or kill the Spawning Pool, Baneling Nest and Evolution Chamber? Spore Crawlers are quickly being built (ignoring the possibility of simply attacking those), so time is of the essence. Remembering the concept of the three pillars that supports every build, which should the Banshees target?
Hopefully it’s obvious that, even though they would be cheapest to replace, the Banshees should target the 3 Tech Structures. There’s no point in killing drones, against 2 hatcheries that would be like spitting into the wind. It would be like punching your enemy’s arm instead of kneeing their groin. Don’t hit your enemy where they are strongest.
Killing the army also will not yield an advantage because, not only are lings and Queens quickly reproduced, but 2-port Banshee leaves the Terran army weak as well. Attempting advantages in two categories results in a weak lead that is quickly eaten up by the opponent’s much stronger advantage in just one category.
Killing the Lair is a possibility, but that target is divided between being a tech and economic building, and as a tech target it’s not a strong one. The natural can always be turned into a Lair as well. Really, the Banshees need to pull the rug right out from under and take out the Spawning Pool, Baneling Nest and Evolution Chamber.
Should that happen the Terran would have the game won, maybe not instantly, but the lead would be cemented. The Banshees (still being produced) would have full map control, preventing further expansions while the Terran could take any base they wanted. Zerg dares not move out before the Pool and Chamber are replaced and the Spire completed, because every ling they lose is lost forever and Terran still poses the threat of killing the irreplaceable Spore Crawlers/Queens with overwhelming Banshees.
Killing the Tech puts the Zerg in heavy defense mode, concerned with only rebuilding their crumbled third pillar before the whole edifice collapses and the build falls in on itself.
Let’s take this concept to another level. Due to race mechanics every race is strongest in one of each of the three categories.
Zerg:
Strongest: Economy
Weakest: Tech
Every good Terran/Protoss knows to never attempt to outmacro a Zerg. Unless there is a major skill difference it simply will not work. Zerg replaces their army the fastest, takes the map the easiest and builds workers with enviable speed.
However Zerg only creates one building per tech. While Terrans sacrifice Barracks without batting an eye, losing a Spawning Pool, Hive or Spire will devastate a Zerg. This is why a Zerg who loses their main has lost the game, regardless of how many expansions they have.
Terran:
Strongest: Tech
Weakest: Army
With so many specialist units, Terran has the easiest time checkmating an opponent through superior tech. Between Reapers, Ghosts, Banshees, Hellions, Thors and Battlecruisers, Terran tech is most feared.
On the other hand Terran production is the weakest, and often all it takes to kill a Terran is clearing out one army. Zerg may constantly trade armies for little gain, but Terran feels every unit lost, especially the ones beyond marines, marauders and hellions.
Protoss:
Strongest: Army
Weakest: Economy
While a Protoss may not replace their army as easily as a Zerg they build it faster than a Terran, it has strong mobility and it is much, much harder to kill. No one worries about a chronoboosted Nexus. All fear the deathball.
On the other hand the Protoss economy mechanic is the weakest. When the game is played properly (and balanced properly) Protoss will be behind the other races in income. This is why an unexpected cloaked Banshee is far more devastating to Protoss than Zerg.
To conclude let’s run down some quick situations of how to hit your opponent where they are weakest:
- If the Queens are sniped the next target should be clearing the map of creep tumours.
- Expand heavily against a Terran mech build. Do not try to “out-tech” it with Immortals/Void Rays/Ultralisks/Infestors. Instead, wear it down with a constant stream of tier 1 units.
- Do not aim to pick off small numbers of workers once the opponent is on 3 bases. Instead, prevent future expansions.
- Expand when you have map control.
- Either get full +1 upgrades ASAP or no +1 upgrades at all.
- The best responses to a 4-gate are cloaked units or units with high mobility, to hit the probe line and get out before the army arrives
- Immediately attack an opponent if you see them start to build at a higher tech. Hit Zerg the second you see mutas, Terran the second you see Banshees, Protoss right after you clear up any DTs. However, whatever you do, DON’T attack if they’ve been sitting on the tech for a while.
- A player is most vulnerable to a heavy attack, not light harassment, within the first few minutes of claiming their natural.
- Base trade race? Escape your workers. Make sure you kill all of theirs. Don’t be too concerned about setting up a new base for harvesting. Base trades involve strong armies decimating tech and economy. The sooner you get your tech back up the sooner you’ll be able to replenish your army.
- Take note of which tech structures you are able to pick off and adjust your army accordingly. Killed a Spire? Build 4-6 Vikings and go kill some Overlords. Killed a robo facility before the first Colossus was built? Get cloak for your ghosts and upgrades for your mmm. Emphasize Medivacs over Vikings. Do not expand. You haven’t killed their econ, you killed their tech, so make sure you recognize where the deficit lies and exaggerate it.
- Just held off a baneling bust? Attack fast. Don’t worry about overwhelming the Zerg defenses, just snipe as many Queens and Drones as possible.
TL;DR
Since games are won through an advantage in either army, tech, or economy, and advantages increase exponentially, it's crucial to learn to attack your opponent where they are weakest.
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EDIT:
So I received many positive responses to this post, and many negative ones. While I don't have the time to respond to every criticism I went overboard in preparing a response to the main complaint: that my Banshee example was wrong. Drones should ALWAYS be targeted over tech structures.
Here's a replay pack where I did my best to replicate the scenario on Shattered Temple:
http://www.mediafire.com/?g7dqk378gv845f2
To quickly reiterate: if you're doing a 2-port Banshee build and you catch your opponent off-guard with insufficient tech do you: kill 20 drones, or kill the Spawning Pool, the Evo Chamber, and other tech buildings?
In this pack are 6 games where I caught the opposing Zerg offguard with cloaked Banshees. All the opponents were in Diamond. I'm in Masters, but (as you'll see from my crappy play) I really wasn't used to this build so I feel we were on even footing.
The first thing I would like to point out about these games is that my example is insufficient in that every opponent I played had either an infestation pit or spire up before I got my banshees there, while the example I created assumed the Zerg was still building their Lair, so the Zergs had actually invested quite a bit in technology, thought not as much as I had, which is the point of the example; that if Terran has a tech lead they need to further that tech lead by targeting tech buildings rather than economy.
This is also why I didn't save any replays where I was unable to establish a tech lead. There was a killer game against a Master Zerg I ended up losing because he transfused his Spawning Pool barely enough times to keep it alive. I didn't feel that was an accurate depiction of the principle I was stating since I wasn't able to keep the tech lead and he was able to kill my sky force with a crazy amount of Corruptors.
Also, in most of these examples I was not always able to fulfill the scenario in that I wasn't always able to kill the Spawning Pool, Spire/Infestation Pit AND the Evo Chamber. Sometimes I just had to take what I could get. However, NEVER did I target drones, that is, until the 6th game, where I did nothing BUT target drones and queens, but I'll speak to that later.
What I want you to pay attention to is the situation that I have created by targeting tech buildings. The Zerg immediately becomes paranoid. They try desperate ling rushes and they stop trying to get map control. They're afraid to pressure with mutas, because with the Spire gone, the only mutas they have are the ones they already made! And with a couple vikings added in, the mutas were nothing to worry about.
With the Spawning Pool dead I could target Queens, knowing they couldn't be replenished. Ditto Evo Chambers and Spore Crawlers
Always the Zerg was ahead in economy. Often they were ahead in army. Never were they ahead in tech (or I just didn't include the replay), and you know what? That tech made all the difference. If they base traded (as happens in a few of the replays) I just lifted my buildings and rebuilt. If they ling-rushed I just lifted my depots. They had some successful counter-harassment, but my superior Terran tech abilities kept me alive. It doesn't matter how many lings they have if they can't get past your depot wall.
The 6th game is the one where I really drove the point home by targeting Drones/Queens in an effort to do ECONOMIC damage rather than TECHNOLOGICAL damage. You'll quickly see how fruitless the effort is. I literally killed 30 drones and 2 Queens with 4 Banshees, and not 1 minute later they were all replaced and the Zerg had built up enough of a muta force to take control of the map. I carried out the game as long as I could but it was very evident that my Viking production simply could not keep up with the muta production. Why not just use marines? Because I was doing a tech build, not an army build, and marines wouldn't have worked anyways Even after the Banshee harass the Zerg was just too strong. Watch the replay and you'll see how little impact my Banshees had. It was like trying to stop a tornado by blowing real hard.
I knew it wouldn't work, but I had to try anyways just to make the point: Zerg's are strongest in economy, Terran's are strongest in tech. Terrans win the game by exploiting a tech advantage, not by trying to create an economic advantage through a tech build.
Sure, Terrans can try an army build, where they focus on army and try to win that way (if the Zerg focuses too heavily on Economy and gives them the army advantage), but the true "checkmate" scenario comes in the first 5 games I played, where no matter how strong the economy, how large the army, Zerg just cannot compete because they lack the ability to damage Terran tech.
I learned this lesson while creating my hellion/Thor build, which starts with a blue-flame hellion rush right before mutas pop. It drove me insane with frustration, killing an entire mineral line's worth of drones, only to have the Zerg transition basically unharmed into the mid-game. Then I learned, I was hitting Zerg where they are strong: economy. I still use the build but it doesn't rely on creating economic advantage, it relies on creating a tech advantage designed to take map control, as hellion/Thor cleans up muta/ling real good.
Also, please no comments on my crappy play. I know I wasn't perfect. I haven't used cloaked banshees against Zerg in a long, long time.
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Edit:
Here's a replay from Liquidwater v RootSlush.
http://www.sc2replayed.com/replays/171722-1v1-terran-zerg-metalopolis
Water does the EXACT 2-port Banshee v Zerg build I was trying to create but does it much better, against much better opponents (and here I thought I was being so original). This is from Water's recent replay pack and this is the build he uses for all of the TvZ's.
He wins by using his Banshees to target tech buildings. First he clears out the Queens, then hits the Spire, Baneling Nest, Spawning Pool and Lair. I think he killed no more than 2 drones. He loses most of his ground army, is definitely behind in economy, and wins through tech. I could not have asked for a better illustration of what I was trying to say.
I realize that haters are gonna hate, but I don't think I could provide any more evidence without being redundant. The goal of a Terran tech build in TvZ is to destroy the Zerg's tech, not their economy.
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Edit: Concerning the suggestion for Zerg to tech quickly against a Terran who FEs: Check out Idra v Select in the IPL.
+ Show Spoiler +
On XC Idra gets a fast Spire and rules the map with muta packs. Select techs to Thors and tries to zone off areas but since Idra's tech advantage gave him map control he was able to expand as much as he desired and capitalize on Zergs strength: their economy.
On ST Select opened reactored Hellions and used his tech advantage to harass the Zerglings and Drones, in other words trading a tech advantage to mitigate an army and economy defecit. Of course it doesn't work. The only way a tech advantage will gain a lead is by furthering the advantage. But hellions can't do that because 1 for 1 they're literally worse than SCVs at killing buildings, which is why hellion openings provide the briefest advantage. As soon as Zerg techs up as well (to Roaches or Mutas) Terran's advantage has disappeared.
The proper follow-up to a hellion opening is to continue to Thors, because that continues along the tech path and increases the advantage, but that's not what Select did. After the Hellions he went right back to marine/tank, but it was too late. Idra had too great an economy/army lead and Select's early tech lead had long-since expired.
Then Idra GG'd. He had one bad engagement and ragequit, even though he was totally ahead. Good ol' Idra.
On TA Select had an ineffective hellion opening. Idra responded with fast Spire but Select teched to Thor, then stopped at 2 or 3, instead transitioning to mass marines. Basically he abandoned he had the tech lead but abandoned it to concentrate on army, but Idra was ahead in that department and the Zerg army game is stronger than the Terran's so he just rolled him.
SP was a simple 2 rax pressure win (Select's so good at that!).
On Crevasse both players expanded quickly and Idra did his trademark quick Spire (as I suggest should be done against a Terran who FEs). Then Select keeps his army low and techs to Thors, but Idra was faster and had Mutas in Select's natural before Select's Thors were out. This is EXACTLY what I suggested in the OP. The proper response to a Terran FE is a fast Spire so that a tech advantage can be created and exploited.
In TvZ the Zerg MUST trade the tech advantage for an economic advantage, because they are the economy race and Terran is the tech race, so staying with a tech build will lose in the long run. There is also the possibility of trying to lengthen the tech advantage by killing Terran's tech buildings but that only works when the goal is to end the game quickly as Zerg tech advantages are fleeting.
Idra was smart and went right for the Terran economy. Then he harassed, picking off loose targets, even the Armory. Unfortunately it was too late to slow the main push, as Terran had enough Thors by then. That main push came when both armies were equal, but the Zerg tech advantage was suspended, since Idra was investing in Brood Lord/Infestor tech but hadn't produced hardly any of either. Select overran the first army with his superior Terran tech but Idra regained the tech advantage when Brood Lords popped just in time to stop the remaining Thors from overrunning his main. Later Idra researched neural parasite (furthering the tech advantage) and overran the remaining Terran forces.
This leads to more important points. People will recognize that Brood Lords and Thors are both late game tech and that Terran tech is stronger than Zerg tech, so why do Brood Lords beat Thors?
First, it's because Thor air attack sucks. Really. It's terrible. There are still imbalances in the game that need to be accounted for.
Second, not ANY tech will beat ANY equal tech. Ultralisks won't beat Banshees. HTs won't beat DTs. Brood Lords won't beat Corruptors. Standard common sense still applies, in this case Vikings are the correct tech to fight Brood Lords.
So there you have it. Idra rolling face by beating Terran FEs with quick Spire. I couldn't have asked for a better illustration.
On ST Select opened reactored Hellions and used his tech advantage to harass the Zerglings and Drones, in other words trading a tech advantage to mitigate an army and economy defecit. Of course it doesn't work. The only way a tech advantage will gain a lead is by furthering the advantage. But hellions can't do that because 1 for 1 they're literally worse than SCVs at killing buildings, which is why hellion openings provide the briefest advantage. As soon as Zerg techs up as well (to Roaches or Mutas) Terran's advantage has disappeared.
The proper follow-up to a hellion opening is to continue to Thors, because that continues along the tech path and increases the advantage, but that's not what Select did. After the Hellions he went right back to marine/tank, but it was too late. Idra had too great an economy/army lead and Select's early tech lead had long-since expired.
Then Idra GG'd. He had one bad engagement and ragequit, even though he was totally ahead. Good ol' Idra.
On TA Select had an ineffective hellion opening. Idra responded with fast Spire but Select teched to Thor, then stopped at 2 or 3, instead transitioning to mass marines. Basically he abandoned he had the tech lead but abandoned it to concentrate on army, but Idra was ahead in that department and the Zerg army game is stronger than the Terran's so he just rolled him.
SP was a simple 2 rax pressure win (Select's so good at that!).
On Crevasse both players expanded quickly and Idra did his trademark quick Spire (as I suggest should be done against a Terran who FEs). Then Select keeps his army low and techs to Thors, but Idra was faster and had Mutas in Select's natural before Select's Thors were out. This is EXACTLY what I suggested in the OP. The proper response to a Terran FE is a fast Spire so that a tech advantage can be created and exploited.
In TvZ the Zerg MUST trade the tech advantage for an economic advantage, because they are the economy race and Terran is the tech race, so staying with a tech build will lose in the long run. There is also the possibility of trying to lengthen the tech advantage by killing Terran's tech buildings but that only works when the goal is to end the game quickly as Zerg tech advantages are fleeting.
Idra was smart and went right for the Terran economy. Then he harassed, picking off loose targets, even the Armory. Unfortunately it was too late to slow the main push, as Terran had enough Thors by then. That main push came when both armies were equal, but the Zerg tech advantage was suspended, since Idra was investing in Brood Lord/Infestor tech but hadn't produced hardly any of either. Select overran the first army with his superior Terran tech but Idra regained the tech advantage when Brood Lords popped just in time to stop the remaining Thors from overrunning his main. Later Idra researched neural parasite (furthering the tech advantage) and overran the remaining Terran forces.
This leads to more important points. People will recognize that Brood Lords and Thors are both late game tech and that Terran tech is stronger than Zerg tech, so why do Brood Lords beat Thors?
First, it's because Thor air attack sucks. Really. It's terrible. There are still imbalances in the game that need to be accounted for.
Second, not ANY tech will beat ANY equal tech. Ultralisks won't beat Banshees. HTs won't beat DTs. Brood Lords won't beat Corruptors. Standard common sense still applies, in this case Vikings are the correct tech to fight Brood Lords.
So there you have it. Idra rolling face by beating Terran FEs with quick Spire. I couldn't have asked for a better illustration.