Kill switchs are normally used when a plane has a gasoline engine in it. The kill switch is actually a switch that will disrupt the ignition system of the engine so that it will not run. Some people will put this kill switch on a plane controlled by a servo so that they can kill the plane with the radio, while others put them on the plane as a seperate switch that is manually activated. These can be very important on gas engines for safety so that the engine can't accidentally start running if it's turned over.

Ken