When your penis drops down after emerging from a vagina or other crevasse after you go to town on her (or him if you're into that).

The joyful feeling after you've cum and your penis can finally rest, whilst still being fully erect like a racing horse.

Got the covid-19 vaccine

When a male ballsack slaps the inner thigh. It can usually be done on purpose with the legs spread and producing a swinging motion.

One can do it to another in some cases when the leg is kicked so hard that it acts like the newton pendulum.