O.Henry makes a joke about a guy who could "skin the cat" on a gym bar twice without changing hand grip. I can't remember exactly what "skin the cat" consisted of. I found many sites about gymnastics, including one that had action figures performing many of them, but none that described it.
Can you tell us what it means?

Well, you would appear to turn something inside out, so I'll guess that you get up some speed going around the bar and then, with arms about two feet apart and straight, shoot your feet in first through the hole. But you'd have to move your hands on the bar unless you were very much double-jointed.

I really don't know, but thought it would be fun to guess.