Every manner of pull-up has its diehard fans. Wide and narrow grip, single and double suspension points, wide handle, rotating bar, slow, and behind the head all have their staunch supporters.
The default CrossFit pull-up, however - a violent, kipping, "anyhow" pull-up - has few supporters even among pull-up connoisseurs. Ours has always been the "cheating" pull-up.
Kipping comes in a myriad of styles, and each athlete has a signature kip, but in its most elegant form the kip is a transference of movement first generated in the horizontal plane, where it comes cheap and easy, to the vertical plane, where momentum and a perfectly timed pull from the back launch the athlete forcefully upward.
This "cheat" derives from a powerful and athletic reversal of hip direction - like that of the clean and the snatch - and expands the primary movers from just the back and arms down through the torso and hip to include the power zone. Far from being a cheat, kipping is a gateway skill with functional utility on the rings, parallel bars, high bar, and floor (the quickest way to get to your feet). Where most athletic communities avoid the kip, we go to great lengths to teach and learn it.