First things first, specific to the images displayed I do not see significant alterations in the external rotation of the athletes pictured (just so that is said). I do see more of a laying back of the torso specific to foot positioning. This position is poor for hitting as we lose significant links to the kinetic chain. Essentially our anchor occurs in the low back instead of linking to the hip and ankle. I also do not see the opening of the pelvis we want to see in the athlete pictured on the bottom. This eliminates the same linking of the kinetic chain specific to the rotational power needed for the same activity. The top athlete is using this rotation, granted in the arched back position, and I would bet has greater range in her attacking ability (meaning where she is able to close and place the ball effectively). From a skill development standpoint the athlete on the bottom could very well be capable of doing this effectively and just be pictured in the incorrect posture for this single instance but experience tells me this is not the case (even when taking a "straight line" striking angle good hitters will take advantage of the rotational power needed as a way to gain valuable ROM).
Good Example of Powerful Jumping/Hitting Position with the Requisite Shoulder Mobility
Beyond that, and more specific to the content of my friend Sam's post, I have to say that the primary way that we communicate effective use of the shoulder is through effective use of the rest of the body. So what Sam communicated is important but for what I believe the majority of my role encompasses is presented in a fairly limited scope (essentially becoming a programming consideration and not necessarily a coaching cue). Within my practice the work is in the coaching and in getting the shoulder work done.
Developmentally speaking volleyball players are more limited in the volume of their overhead striking as there are many obstacles to doing this effectively at the youth level. So volleyball teams and athletes essentially have to earn the ability to attack aggressively and this tends to match up well with what they are prepared for as an athlete. I believe a major roadblock in this process is coaches doing private lessons or hitting clinics where they are teaching overhead actions the rest of the body is ill prepared for. So I don't go on a rant about this that takes this post further off-topic the easiest way to initiate some of the physical actions we want to impact overhead work is through the dynamic warm-up and in brief jumping and reaching progressions that teach athletes to complete vertical extension and close aggressively to a great landing position.
Many of the progressions I outlined in the 'Vertical Jump Methodology' post can serve as effective ways to vary the skill and effort. The need for variation is there because simple exercises only guarantee improvement in said exercise and the primary physical capacity it develops. There are skill development considerations that need to be adaptable to a constantly changing environment (in the video posted below what was she supposed to do?) and to me, it makes no sense to develop great vertical power, rotation, or upper body power and then attempt to apply it to the skill. It is more than just a high contact point, it must be integrated, and it must be practiced.