It's hard to explain what it is. I'll try to use an example. You know when your workers mine minerals they can be on top of each other? But when your workers attack something, they can't be on top of each other. Well this trick lets your workers attack a unit or building on top of each other. Exploit your workers to your advantage! How to do the trick: Select a group of workers(Drones, Probes, SCVs), preferably a large group. Hold down Shift and Right click an empty Vespene Geyser 3 or 4 times. One time won't work. Hold down shift again and perform a command like attack on an enemy's building or unit. After all the workers went to the Vespene Geyser, they will move to where you told them to or attack to where you told them to in a sliding motion on top of each other. For a better understanding, watch replay.