Without an overlord, mutas will spread out quickly in a line formation.
With an overlord, mutas will spread out slowly in a small circle.
Here's a video where I demonstrate the differences:
Performing this technique is very simple:
1. Group an overlord (or any other unit that moves) with your mutas.
2. Fly your mutas around by clicking near your group.
Mutas will group up more, allowing you to more easily single out enemy units. Because the group is smaller, you will be able to more easily dodge enemy units/turrets.
I really see no significant downsides to this technique. It's still not as good as Brood War muta stacking, but it's better than nothing. Obviously you wouldn't want to do this against thors or other units with splash, but you can just take the overlord out of your group once they come into play.
Edit: Apparently Nestea has been doing this for quite a while. People probably thought he was trolling. =)
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Phoenixes also stack better with an observer added to their group, which is helpful when microing against mutas. Just make sure you watch out for infestors!