This is typically called “full” external rotation. I wouldn’t say “extreme” although it’s about as far as it can go. Chris is absolutely right when he says that this position is not possible without the momentum and action of the rest of the body. You can’t do it if you just try to lay the forearm back.
If you don’t get to this position at the right time, you will not likely throw as hard as you possibly can. Almost every time someone says to me that they are just not getting any velocity, the commonality is that they don’t get to “full external rotation”.
Now, this is really a paradox. That which begets velocity is what is most likely to result in injury to the rotator cuff and/or the UCL. That’s the dilemma for pitchers and their coaches. On one hand, we try and try to develop mechanics that maximize power and on the other hand, we try and try to find ways to reduce the likelihood of injury. They’re almost mutually exclusive. Just check out the stats re: pitchers on the DL and those that have had shoulder or elbow surgery.
Long story short, this picture does not represent an unusual or extreme condition. ALL, and I mean ALL, hard throwing pitchers get to full external rotation at exactly this time. Also, it is VERY stressful on the elbow and shoulder. What to do, what to do???