It is a button that when pressed splits your units so they don't bunch up as much (a uniform spread). Basically, if pressed, the units start to move apart to a certain distance whilst either the units are standing still or moving (or move attacking). The speed and distance of the split would be subject to balance. You can also shift click the split button. However, if another move or attack command is issued after pressing the split button then the units will start to bunch up again.
What does this do?
Obviously AOE becomes less effective. Basically, your units are going to be spread out more often than they currently do and it will be less apm intensive to do really great splitting (some AOE attacks and spells might need small buffs).
It spreads out the battlefield. I think the biggest change will be where players can now spend their APM. Engagements will be less focussed on splitting armies and instead on the use of unit abilities and multiprong attacks.
Reduces the DPS of battles (AOE less effective and less units will be firing at the same time).
An example of how it would play out
The famous mmmm/bling muta fight. One of the best examples of where splitting matters and is seen as quite the spectacle when both sides are splitting to avoid the splash damage of the opposite side. I believe what would happen is that the players will only spend a few clicks to split their armies instead of several clicks (the distance and speed that the split happens shouldn't be so high that you can't still spend some APM to make the split even better). The extra APM could now be spend on having more tanks/widow mines targetting on banelings or mutas, or perhaps using that APM on a drop of marines to make them more effective. The zerg could spend their apm on having seperate groups of banelings follow groups of marines, or using the mutas to pick off medivacs, or manage ling runbys better.
Why I think it's better
It makes surprise AOE way less painful and managing against surprise AOE a lot less APM intensive for lower leagues. For example, in low leagues you have a disruptor come out of the fog of war whilst your building your supply depots back at base and then you look back and half your army is gone. This situation is frustrating and pre-splitting your army while you go build some supply depots means that your macro goes to sh*t because your spending your APM presplitting.
It creates more interesting battle dynamics where players split too much and they lose dps in the fight because not all their units are now shooting or maybe they don't split enough and get wrecked by AOE.
It makes the battles last longer because the DPS is reduced.
You can feel a bit more like a pro! Sc2 introduces easy macro mechanics and smart casting. These two mechanics made you feel like a sc1 pro. I think Blizzard should take it a step further and help players split too.
Doesn't this lower the skill ceiling?
Yes, but basically everyone is complaining this game is too hard. Players can now focus more on the abilities of their units, macro and multiprong attacks more.
How much space is between the units and how fast do they spread?
That would have to be tested and possibly balanced seperately for each unit.
How does it feel across all the leagues?
I think that it removes a lot of instant death frustration for lower league players (bronze to platinum). The APM of platinum to masters players can now choose to spend their APM splitting more to further reduce AOE damage or using unit abilities and keeping more on top of macro. High masters and grandmaster players will have their APM freed up to do multiprong attack/defense and use unit abilities.
Isn't this just the same as reducing the radius of splash damage?
No, because it's essentially giving easier counterplay to splash whilst still giving the ability of splash units to still do terrible, terrible damage.
Why not have the units permanently spread out?
This has been discussed to death so I'm not going to talk about it here. I think a button that spreads/splits units creates more dynamism.
Poll: Is it a good idea?
Terrible idea (31)
94%
Great idea (2)
6%
Not sure (0)
0%
33 total votes
Great idea (2)
Not sure (0)
33 total votes
Your vote: Is it a good idea?