The Active Brake II mosfet is designed to stop your AEG from stopping in semi mode with the spring partially compressed. It makes sure the motor doesn't overspin and cause un-needed strain on the gearbox.
 
this product is a great low cost alternative to an optical mosfet being 14.8v lipo ready and increasing rate of fire however do not attempt to install this on your own without expert knowledge as requires you to disasemble and reasemble gearbox and use of a soldering iron. I recommend use of my mosfet installation service