Hitting the porcelain.

It is the act of hitting ceramic material with urine (usually the inside bowl of a toilet), so that you don't make any noise when you pee. More common amongst males than females for obvious reasons. It can be done whilst standing up or sitting down. It can also grow on you as you may start to see it as some sort of game.

Wife-"Honey, where were you?"
Husband-"Just using the restroom."
Wife-"How come I didn't notice this?"
Husband-"Well, I was hitting the porcelain."