The idea that a golfer has to externally rotate the right humerus in the early downswing to avoid flip-rolling has no biomechanical validity.
It simply reflects a lack of understanding of golf swing mechanics/biomechanics.
Consider Hunter Mahan's downswing action.
Here are capture images from his video.
Image 1 is at P4, image 2 is at P5, image 3 is at P5.5, image 4 is at P6 and image 5 is at impact.
Note that his two elbows are level at P4. Note that his two elbows remain level during the power package slotting phase between P4 and P6 - and note that there is no external rotation of his right humerus at any time point during his downswing action.
His P6 position is perfect and there is no biomechanical reason why he cannot avoid flip-rolling through impact. I think that if one concentrates one's attention on the 3-D motion of the LAFW, one can readily understand why external rotation of the right humerus is not an obligatory requirement in the downswing. It only has the advantage of getting one's right elbow deeper down to a pitch location in front of the right hip area and thereby facilitates the maintenance of maximum lag for a longer period during the mid-downswing.
Consider this KM-article -
www.aroundhawaii.com/lifestyle/health_and_fitness/2011-11-pga-tour-clubhead-lag-part-2.htmlKM states-: "
What is important about this move is that the external rotation of the right shoulder is primarily responsible for “dropping into the slot” or flattening your downswing plane."
First of all, it is not a right shoulder that is externally rotating and it is the right humerus that is externally rotating. Secondly, it is true that it helps to shallow the clubshaft plane during the power package slotting phase, but it doesn't mean that one cannot slot the power package correctly without external rotation of the right humerus (see Hunter Mahan's downswing action above).
KM then states-: "
Of course, you can do this incorrectly. If you pull the right arm forward while internally rotating the right shoulder, this is what it looks like...This explains why Brian’s right arm gets stuck behind. The muscle mechanics involved are pretty simple. By internally rotating his right shoulder, he is using this powerful move way early in the downswing. But once he uses this move, he cannot continue to use it. It will eventually run out of range of motion. Then the only available motion to the right arm will be to extend it early (which he does) but in golf lingo isn’t that what we call a cast or early release? Yup."
I think that's BS! KM is referring to an animated image of Brian Gay, and like Hunter Mahan, BG doesn't externally rotate his right humerus in his early downswing, and the two elbows remain level between P4 and P6. However, BG doesn't get stuck!!! Like HM, the right elbow will remain slightly behind, or alongside the right hip area, and it will predispose to an earlier right elbow extension and therefore an earlier loss of maximum lag. That's all - and it doesn't predispose to casting or an early (sweep release). A golfer can still use a random release action of PA#2, but it will occur slightly earlier in the mid-downswing phase.
KM also posts this comparative image of Tommy Gainey and Olin Browne.
One can see that TG has maintained his lag to a greater extent and his right elbow is more in front of his right hip area. KM may label TG's position as reflecting more external rotation compared to OB's position (see green lines). However, I think that the primary difference is that TG is adducting his right arm towards the center of his body faster (compared to OB) and thereby getting the right elbow to be inside the hands at P6, and that active right arm adduction motion
secondarily produces external rotation of the right humerus.
KM then posts their impact positions.
KM then states-: "
Firing the internal rotation of the right shoulder is an automatic trigger to fire the rest of the segments. The key is firing it as late as possible. Firing early will yield a flip."
Again, OB is not flipping into impact and flipping is
not more likely to happen because the right arm/forearm/wrist motion is not the primary biomechanical factor that determines the pattern of motion of the LAFW and clubshaft between P6 and P7 in a swinger. A swinger is pulling the club with his left hand secondary to the release of PA#4, and the right arm/forearm/wrist only play a
secondary role in synergistically assisting in the release of PA#4 and PA#3 in the late downswing between P6 and P7. Even if the right arm/forearm/wrist motion is passive between P6 and P7, a swinger should have no problem completing his release of PA#2 and then PA#3 without any flip-rolling problem if he maintains the forward motion of his FLW through impact due to the efficient release of PA#4. I don't think that a swinger should ever think of "firing" his right side because that can predispose to flip-rolling. The right palm should keep up with the FLW between P6 and P7 and there can be a small sense of push-pressure felt at PP#1, but the right arm/forearm/wrist motion should never be driving the FLW forward (faster than the FLW is being driven by the release of PA#4) in a swinger who swings the LAFW into-and-through impact.
In summary, right humerus external rotation in the early downswing only helps to shallow the clubshaft and helps to maintain maximum lag for longer during the power package slotting phase, but an absence of external rotation of the right humerus doesn't cause flipping in a swinger who swings his intact LAFW down the inclined plane between P4 and P7.
Jeff.