[yuhaoyang]

depending on which model you get, stuff like active braking, fire mode programming, ROF programming. It essentially bypasses the trigger mechanism as a method of directly completing a circuit, preserves your trigger contacts, and gets rid of the annoying issue of over-cycling the piston (I'm kind of OCD about that =o). It is marginally beneficial to your motor as well.
Also you can set it between water gun and RS ROFs, and burst fire, which is nice.