I have no idea why you state that Stuart Appleby is not applying extensor action throughout his backswing. His left arm is continuously straight throughout the entire backswing and the straight left arm acts as checkrein on his right upper limb's movement. Although the major right humeral movement is an external rotation of the right humeral head within the right shoulder socket (while the right shoulder socket is moving back to the TSP), there is always a small amount of right arm abduction occurring at the same time. Abduction of the right arm (while the right elbow is bending) is responsible for extensor action. I think that the amount of right arm abduction required to apply extensor action depends on the checkrein action of the straight left arm complex - which consists of the i) the left arm and ii) the amount of "give" within the left shoulder socket (amount of attainable laxity between the left humeral head and the glenoid fossa) and iii) the amount the left scapula slides forward during left shoulder rotation. Golfers who have great flexibility (great ability to slide the left scapula forward and great ability to widen the space between the left humeral head and the glenoid fossa) will require a greater amount of right shoulder abduction per unit time - in order to maintain a constant level of extensor action throughout the backswing.
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