Well, I don't know the literal name of the rule, but basically it states, when fighting, you never hit below the other person's waistline.
So heres a scenario:
You were out at some bar late at night. There was a misunderstanding with the really big guy next to you, next thing you know, you're out in the parking lot fighting. Not only is this guy huge, but he really knows how to fight and is much better than you. He winds up to deal a blow that you just know would knock you out instantly and there is no way for you to dodge it. He made a mistake however, and leaves his groin area wide open. If you kick him there, he won't land the blow, and you would have basically won. Do you go for the opportunity?
I, personally, would. Being knocked out is not very fun.
Edit: Added a few more options to the poll
Edit2: I realize that a kick in the balls is not the "I win" button. We'll assume that it IS an "I win" button for the scenario.