If you peruse the injury threads on the crossfit.com board you'll find a lot of shoulder injuries, specifically SLAP tears (tears in the labrum). A lot of them seem to come from kipping pull ups, and I've come across a possible explanation. There is an article explaining what a SLAP tear is and how it comes about, most notably:
In theory, SLAP lesions most likely occur in overhead athletes from a combination of these 2 previously described forces. The eccentric biceps activity during deceleration may serve to weaken the biceps-labrum complex, while the torsional peel back force may result in the posterosuperior detachment of the labral anchor.As some one else pointed out, when you look at the kipping pull up with this knowledge you'll see:
Eccentric bicep loading = the negative part of the pull-up, the arms straightening at the bottom
Arm deceleration = the swing into the kip
Abduction and external rotation = the position of the arms at the apex/reversal point of the kip
Just imagine the "peel-back" force in the shoulders when they are forcefully wrenched back at the apex/reversal point of the kip. If you really think about it, the head of the humerus is trying to push out the front of the glenohumeral (shoulder) joint, the kipper is trying to RELAX their muscles at the bottom to get the most out of their kip, leaving all the strain on the LABRUM (think of it as a suction cup on the scapula that holds the humerus on the body, with the greatest forces on the top/superior and the front/anterior parts), and the resultant forces--over time (repetition)--TEAR the TOP part of the labrum from the FRONT to the BACK...this is therefore a SUPERIOR LABRUM ANTERIOR-POSTERIOR injury mechanism, almost perfectly designed (as evidenced by the orthopedic doctors making a ton of money off of people who do them).
Steven Low, writer of an awesome exercise blog, has chimed in on this subject matter as well. I have much respect for the man. Not only do I have a link to his blog in the margins, but his posts have really helped me in the past with my injuries (led me to find a DO for my hip, helped realize massage would fix the tendonitis, etc.). Concerning SLAP tears, he says:
The reason why you don't seen the top CF athletes getting slap lesions is because their shoulder mobility sucks. If your shoulder mobility sucks when you relax your shoulder muscles at the bottom of the pullup you have your muscles contracting against opening the shoulder all the way thus mitigating any forces on the biceps long head tendon. In addition, the muscles do help bar forces from the shoulder.There's also another article here by Whole 9 that basically sends the same message of do dead hangs before working on the kip. Crossfit Virtuosity also recently mentions how important it is to have strict pull ups before you start kipping (and provides a great program to get strict pull ups).
However, when you get people who are weak (e.g. women in general, or those with good shoulder mobility) they're the ones getting the primary forces distributed through the muscles into the other soft tissues such as the long head of the biceps. This is especially the case with learning the kipping pullups where there's going to be a lot of jerking down into the motion while learning it.
I would never ever ever ever recommending learning kipping before you at least have 3+ deadhang pullups. It's not just not a good idea putting trauma on the shoulder especially when it's easy to not do it right and jerking down into the movement which is much more dangerous than a smooth movement.
Jerking down from toes to bar can do the same thing for reference.
This kind of concerns me because my shoulder mobility is pretty darn good, however I have a very smooth swing, without a jerk. I also wonder if butterfly exposes the shoulder in the same way (don't think so). But from now I will definitely make sure I'm keeping my shoulders engaged at the bottom of a kip, I already have a tear in my hip labrum, and I really don't want to add a shoulder one to the mix.
...I should also point out that, as far as I know, no one at CFOT has suffered a SLAP tear. This is further testament to the training and programming that Jerry lays out for us. The man knows his stuff.
(UPDATE #2: Turns out I have a tear in the back of my shoulder's labrum, not a slap tear, but a tear nontheless. I've decided to forego surgery and try prolotherapy to repair the damage naturally. For more about prolotherapy check out these two videos here and here. I've started a vlog where I document what recovering from prolo is like: