Basically when someone was standing at the urinals you would sneak up on them and try to pull there trousers down to there knees or ankles...now i guess the joke was the victim is stuck standing there not able to pull there trousers up because there in the middle of having a pee and looks like a little boy going for a wee ....I'm sure every boy remember being 3 years old and you pull your trousers and underwear all the way down to your ankles when you went for a pee.
I would say this was probably done to almost everyone in my school at some point regardless if they wore pants or boxser.
But when I reached year 10/11 the boys who still wore pants/briefs were offen the main targets.
I even had my pants pulled down before literally someone pulled my trousers down to my ankles in the busy toilets then pulled my pants down to me knees