R_J_K75
Legend
If playing in a system with critical hits, I would allow such a thing on a critical hit. It wouldn't do any more than the regular critical hit would, but the story impact would be memorable.
I can speak from experience, getting kicked square in the nuts hurts really bad, and also stuns you if not outright drops you. In a street fight, if you get hit there there is a good chance that the other person is going to get the upper hand. So I would say that the effects of being prone or stunned in this situation outweigh any damage. Of course theres some sick and twisted folk out there who like the old Rochambeau.