A killswitch is exactly what you think it is: A switch that kills an operation in an emergency situation when the operation can't be shut down in the usual manner. It shuts your operation no matter what the consequences.
You can actually craft a car security killswitch to prevent a thief from taking off with your car. Check out this video to learn how to make a killswitch. This method is inexpensive and you'll need to get into the ignition valve.