I have this image of you writing this email with your garage filling up with exhaust fumes.
How much fuel is there left in the tank?
I've attached a pic of my magneto. There are different types of end caaps and bodies though. Mine is a manual advance retard, and that shiney button you can see in the middle of the end cap, earths the magneto and makes the bike stop.
More sophisticated versions have them on the handlebar with a wire trailing down to the magneto. This means that you can cut the engine without taking your hand of the 'bars, when the throttle has jammed wide open and you're heading flat out towards a bus queue of pensioners.
Cheers.
Iain