Clitheroe said:
I guess I've never noticed that before?

Also, that's probably because only young (>35ish) and highly virile men can trigger that defense mechanism. The muscle isn't strong enough in other cases

@notawerewolf: Actually, the testicles are supposed to stay inside the scrotum. The condition is called "Retractile testicle" and it's an abnormity, albeit a benign one. The external inguinal ring is usually too narrow for a fully grown testicle to pass.