The ignition kill device is a small metallic tab that sticks through the throttle plate with a wire on the bottom that runs back to the ignition coil. On the stop position the ignition coil is earthed out via this device to stop the engine from running.
However over time, these parts wear, insulation breaks down, etc and sometimes you can have it short out when you dont want to.
I have even come across spark plugs that show a good spark when outside the cylinder but loose spark when under compression.
