Muta Magic box

Many Zerg players still see Thors as a hard counter to a Mutalisk build. The Mutalisks' tendency to group up into a tightly packed ball combined with the Thor's aerial splash damage and bonus damage against Light units can make quick work of a pack of Mutas. However, it doesn't have to be that way. A little extra micromanagement with your Mutalisks can give you the edge in these encounters.

It's a tactic called the "magic box." The magic box concept isn't brand new to Starcraft, it was used in Brood War as well, but Mutalisks currently gain a higher benefit from it than any other unit in Starcraft 2 and probably moreso than any unit in Brood War. So how does the magic box work?
If you're a Zerg player that has used Mutalisks, you have surely noticed how eager they are to pile on top of one another. When you give them an attack command, they all fly toward the enemy and stop at the same exact point and bunch up. Since they're aerial units, they are capable of stacking up on top of one another. It's easy to create a situation where all of your Mutas are sent to attack a specific unit and they all start attacking from the exact same spot on the map and then slowly drift apart. If your Terran opponent has a mech build with Thors (rather than an infantry MnM build), that piled up formation puts all of your Mutalisks at risk. They take a staggering amount of excess splash damage simply because of how the units move. Once the Mutalisks have finished drifting apart, the splash damage goes down significantly. Careful movement can prevent that initial volley of splash damage from ever happening.
The goal of the magic box is to let your Mutalisks complete the "drift phase" of their movement far away from your opponent. Once they've drifted apart, they will hold that formation as they fly around the map unless you issue a new attack command or tell them to move to a point within their formation.
Select your Mutalisks (which you've placed into a control group, right?) and click in the center of them to force then to clump up. They will clump and then drift apart. Once they've stopped drifting and are in a wide spread formation, this is the formation you want to keep them in. They will hold this formation as you move them around the map, as long as you take care not to issue a move order within their formation give them a specific attack target. To attack with your Mutalisks, do not issue an attack command. Fly your Mutalisks over your target, much like you would aim Zerglings past an enemy army to surround them, and hit Hold Position. The Mutalisks will hold their formation and automatically attack targets beneath them. If those targets are Thors, the Mutalisks will not suffer any splash damage whatsoever. This works best with a group of less than 40 Mutalisks. Between 36 and 40 is where the formation starts to break down, and they start bumping into each other again.
This technique does not turn Mutalisks into a hard counter for Thors, but it does significantly reduce the threat that Thors pose to your Mutalisks. Getting enough Mutas to take down a pack of Thors is still going to be one of the most expensive ways to try and bring down those Thors. The magic box technique could potentially save you from needing to make a full tech switch, however. If the magic box sounds complicated, don't worry, it can sound much more difficult than it actually is. To see the magic box in motion, check out the tutorial video from PsyStarcraft which is embedded below.