The only time you ever ever cut of the engine via ignition is in an emergency. Never never never ever ever do it. You run the risk of hydraulic lock and destroying your engine not to mention flood the cly walls with fuel and washing away any lubication that is there.
Yes I heard of float plane operators doing this for a dead quick stop on water. But I can't say this more seriously than don't ever ever do it. And especially never do a compete on off on both mags above idle.