ball clapping is when your nuts hit right between your dick and your ass. usually makes a loud noise when hitting, especially when sweaty! Tends to happen during sexual intrercorse, and when jacking off.

when you're hungover as shit after a spending a night drunk as a skunk

The sound it makes when you get hit in the balls, usually meaning there was a good connection.

Ball Clap is when you have just walked out of a shower or your balls are otherwise wet and your balls hit your thigh creating a clapping sound.