'Post micturition convulsion' is the medical term for the shudder you get near the end of a wee. Until I looked it up I'd always called it a 'chilly wiggle'.
and during a heavy impact can act like a hammer destroying the knee and splitting the lower leg bone into 3 or more pieces
A "Tincleton"* is "The sort of person who likes to amuse himself in other peoples toilets by flushing mid-pee and then seeing if he can finish before the flush does" *The meaning of Liff 1983.