Let's imagine that you're just a dumb-assed kid trying to muck around with the timing and idle on your old Grand Prix, having it in gear with the parking brake on, and the parking brake decides it's finally had enough of that crap, so the car starts creeping forward toward your future father-in-law's newly-painted garage door, and you're too brain-dead to just pull the coil wire to kill it. Well, if that happens, don't stand in front of the car trying to push back like Superman, because it just won't work.