is a negative sum game
mainly played by young men to show how rock hard their balls are, and how fast their reflexes
Or, alternatively, because they are into pain.
The game is played as follows.
The two competitors stand opposite with their hands flat together with their middle fingers touching. One person is 'it'. His job is to move one of his hands (never the other- the other must remain touching the opponents hands) and slap the back of the opponents hand.
However, as soon as his hands are no longer together the opponent is free to dodge. if the opponent dodges successfully, then he becomes 'it'. Otherwise the game continues with the same person continuing to be 'it'.
But if the slapee moves their hands away before the slapper has separated their hands, then the slapper gets a 'free slap'; which means that no dodging is permitted for that slap; and any dodging at all is awarded with another free slap.
It's a test of reflexes and anticipation really. There's no defined end to the game, you keep slapping until you cannot take the pain and wimp out. Your only reward is bragging to your friends about how much the back of your hand has swelled up.
I once saw Jeremy Clarkson playing it against motor racing champion Michael Schumacher. Jeremy was able to more than hold his own, which suggests that extremely good reflexes are not a prerequisite for driving extremely fast; Michael Schumacher readily agreed.