Let's see if I can explain in words what I see at the gym daily.
Some dude (a few of them do this really) starts on the chinup bars at a dead hang. Then he pumps his hips forward so now his legs are moving behind him, while still hanging (and his lower back is arched).
Then thrusting the hips backward, he causes his legs and feet to go forward again. The back becomes rounded.
He repeats this a few times until he is basically using himself to "swing" back and forth until his legs can become parallel to the ground in front of him.
He repeats quite a few times.
The movement is relatively fast and does not seem to be any kind of exercise that is discernable to be helpful (but I could be wrong). It seems to solely use momentum.
I don't think it's a warmup because it's a rather violent movement.
Any ideas on what this is for?