Ahh...PBN's...What's the scoop?
Ye olde press behind the neck. It isn't much in favor these days--for good reason. Few US athletes have the shoulder mobility to do them properly. They are too busy doing that "other" press that shall not be mentioned here. I have cringed many times watching burly football players try to finish their summer workout doing PBNs; they have no business doing them. Their shoulder mobility sucks and they wallow in weight training programs lacking full ROM shoulder movements that perpetuate poor shoulder mobility. And so it goes.
That said, PBNs are a staple of weightlifting, but the weightlifter learns not to be dependent on the ability to press--that's an infraction of the rules. Snatch grip and clean grip presses, push presses and jerks behind the neck are used all the time. But these are people who have to put stuff over their head all the time; and the strongest position overhead is slightly behind the ears, just like Shannon Sheesley demonstrates here (on the L) with 80 kg. Carissa (on the R) is struggling with 110 kg here because it is out in front, where she is forced to use her strength to hold it vs hitting the strong, stable close-packed position of the shoulder in maximum flexion/ER. And guess what? Carissa is coming back from shoulder surgery--she had poor shoulder overhead mobility before and injured her shoulder. Now she is back, and has snatched a PR 91 kg and jerked 114 kg (at 63 kg bw) but she has to work like a dog to get and maintain proper shoulder mobility with her overhead lifts.
Personally, I like PBNs and do them all the time with the empty 15 kg bar or more in an attempt to create and maintain full shoulder flexion. I also do headstand to handstand push ups for the same reason. If gymnasts can do it, why not me? Same movement, just closed-chained. But I have built mobility first before attempting any strength, e.g. I have created the proper context. Few athletes need a bunch of strength in this range, but they could sure use the mobility and a basic foundation of strength. For sure, the movement of the bar from the upper back to the overhead position demands a distinct ROM from the glenohumeral joint, the scapulohumeral musculature and the thoracic spine. If the pecs and internal rotators are tight, this movement will be challenging and could present problems.
Furthermore, most weightlifters are not pressing maximal weights from behind the neck. They usually push press or jerk heavy weights behind the neck--and I think this is key: That most challenging ROM just off the shoulders to the top of the head is "bypassed" by the legs moving the bar quickly in an "unweighted" state. Once the bar is above the head, the final press out is easy. In the jerk, the bar may not even move above the head, as the lifter pushes their body under the bar. There is no stress of bearing weight in the hands in the most challenging ROM behind the head.
Here's a tip: When the bar is overhead, the wrists should be pronated, but fully extended, the scapulae depressed and the shoulders externally rotated as much as possible. If the glenohumeral joint is internally rotated, it is bad news. Get your elbows in line with or behind the ears. Play with the PBN movement with PVC pipe or a stick. Feel the difference when you actually externally rotate the GH joint with the empty bar/pipe/stick overhead and your wrists pronated & extended. It is a different movement that few people actually do. I think this is key to safely doing overhead movements. For the weightlifter, it is essential to protect the elbow with good wrist and shoulder positioning when receiving the bar overhead in the snatch.
I would love to hear from anyone who has actually had someone injure themselves with PBNs--neck, shoulders??? I once heard a DI strength coach talk about fearing cervical herniations from this exercise. That seemed a little paranoid. Cervical herniations don't appear to happen in weightlifting training. If we never let kids bench press and everyone did handstands and PBNs from day one, would we even have this question or debate? My guess is probably not. But our collective tight pectorals and lifestyles have us rarely putting our hands above our heads, from day one. And thus, many are ill prepared to do PBNs and would be better off with dumbbells.