I'm no expert in glenohumeral anatomy, but I've noticed that if I do "triceps-style" bar dips, I have no pain nor impingement in the shoulder joint whatsoever. This is the version of dips where the elbow go as far back as possible, almost as if they're trying to touch one another, as opposed to "chest-style" bar dips, where the elbows are much more flared out to the sides.
But, taking a look at the skeletal structures of the shoulder joint, I see no reason why moving the humerus "backwards", as opposed to just flaring it out to the sides, should somehow make the movement a lot more impingement-less (mainly across the AC joint, which seems to be the most likely source of impingement).
Granted, a simple "skeletal/ligament" view may omitting some other important detail as well --- if my elbow is pointed backwards (with scapulae retracted), I can move the elbow upwards only so far (I cannot raise my elbow above my shoulder joint when it's pointed behind me, for example).
So, are there actually other structures in place (e.g., muscle tissue) that could be keeping 'triceps' dips from hurting the shoulder joint? My best guess so far is just deltoids, traps, and the long triceps head simply all bumping into each other thereby blocking any pain-causing or impingement-causing ranges of motion.
Edit: According to the image below, what exactly prevents sagittal shoulder extension a full 360°? Is it the stretching of the deltoids, or some occlusion between the rear muscle groups bumping into one another, or something else? Since whatever it is, it allows me to do triceps dips comfortably with no pain or impingement whatsoever.