what is cringewrthy about this i did it and my arm hand feels ok
Maybe the joke is going over my head, but if you try to do it quickly, you're moving your hand as if you're wanking.
Kinda like the thing where you tell someone that if they pretend to pour salt from a salt shaker into their mouth, they'll taste salt. But in reality they look like they're pretending to suck a rooster.