Sack tapping
Sack tapping
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Sack tapping - Wikipedia
It is also called "nut tag", "bag tag", "sack whack", "bell flicking" and "roshambo", the last name coming from an episode of South Park that featured the practice.
Sack tapping is a slang term for a game where a participant attacks, by slapping, tapping, punching, kicking, elbowing, twisting, or backhanding a victim's testicles. The term derived from 'sack', slang that refers to the scrotum, and the activity is a form of groin attack. This sociological manifestation of bullying can result in severe testicular injury that may require amputation as the only form of treatment.
Anyone can punch someone else in the nuts hard and catch them off guard. The artistry of this time honored tradition lies in skillfully distracting your target and when the time to strike comes your blow must be gentle yet swift.
Yeah, it is the surprise and the relief that it wasn't painful is what makes it tapping and so entertaining. Watching someone curl over, but then realize they were just fooled is what makes it comedy gold.