The black wire shorts the primary side of the coil to a known ground. A parallel connection will not kill the engine if only one switch is closed. You must use a series circuit. I use a sturdy SPST (single pole-single throw) toggle switch from Radio Shack. Simply run the black wire to one terminal and the other terminal to the engine block. You can use a micro switch activated by a servo for the electronic-operated killer.

Dr.1