When you're running down the stairs very quickly in boxer shorts, and the weight of your testicles causes your scrotum to move against your gooch and make a loud clapping sound.
John: Ew, Mike. Yesterday I was running down the stairs and I did a stairclap. My mom heard. She looked grossed out.
Mike: EWW! DUDE, I DIDN'T NEED TO KNOW YOU HAVE A NASTY STAIRCLAP!