I had a friend who had fallen under a tractor-pulled lawnmower as a small child. He didn’t have all the bits on his hands that he was born with. On one of his hands he only had his index finger up as far as the first knuckle. He did a great version of that trick.
He could also tickle the palm of your hand with the stump of his finger while shaking hands, which was disconcerting to say the least.
