When a man has completed urinating and is in a hurry to put the equipment away and get back to whatever he was doing. In the process he gets his "bag" or at least some skin caught in the zipper. The resulting pain is caused by "bag snag".
In a rush to get back to the poker table I suffered a "bag snag" when zipping up.
Don't be in such a hurry or you could suffer from a "bag snag".