Well if thats the case, you may wish to limit whats being done down there a bit, it sounds like your cuaseing actual damage.

I would also not reccomend using things with sharp edges, like certian kinds of belts or rulers on certian areas, (like on or near the uretheral opening) the sides can come down unevenly and then the force of the blow ends up consentrated in a narrow region instead of being more evenly distrinuted.