It is a switch operated from the signal from your transmitter. When the transmitter sends the signal, The Kill will turn the ignition on. The benefit is that you can turn the ignition on or off from your transmitter. Also if your Receiver did loose power the kill will automatically kill the motor. This means you have less distance to walk to pick up the wreckage.

Here is the one I use. http://www.smart-fly.com/Products/Ignition/ignition.htm