What is the Eraser Trick in StarCraft?
Here's a summary of what it does:
"Doing the Eraser trick is very easy, you only need two or more Science Vessels with Irradiate researched. To do the trick, simply have your two Science Vessels irradiate each other, and fly them over or near the units you wish to kill. Since Science Vessels are not biological units, Irradiate has no effect on them; however, Irradiate's splash damage hurts surrounding biological units."
taken from http://wiki.teamliquid.net/starcraft/Science_Vessel_Eraser