It's no surprise to me that pinching the spinner like that works. You're slowing the engine down a little so it "backfires" a bit and kicks back in the opposite direction, which in this case is "forward".

But I totally agree that it's not a great way to go about things from a safety perspective, and you'll understand if I don't try it myself.

In two weekends we had 2 guys spray blood around the pits. Bad things can happen, so be careful out there.