For those unaware, the Magic Box refers a common micro technique used to minimize splash damage on Mutalisks and other air units. Normally, when attack moving a large group of Mutalisks, they tend to clump up and overlap. To prevent this, simply order your group of Mutalisks to stop, allowing them to spread out naturally, then move command them so that they stay in their spread-out formation.
It's common in Muta vs. Thor matchups to allow your Mutalisks to spread out before engaging,, then move commanding them until they are directly above the Thor, allowing each Muta to attack without having to clump
StarCrafts Season 5