Dear tongzilla,
I could say that the golfer first relaxes and hangs the arms in the natural bend of the setup and then structures the body with muscle tone of about 3 on a scale of 1 to 10 in the hands, forearms, upper arms, shoulders, and top of the back, and that this is the form that stays coordinated during the stroke so that the arm pits do not open in the backstroke or forward stroke. This coordinates the lead shoulder with the movement of the putter head both back and thru.
Instead of that, which is all true, let me try this: it boils down to the lead elbow staying in one orientation throughout the stroke so there is no change in relative orientation between upper arm and forearm on the lead side, whether in the backstroke or the forward stroke.
You can experience this artificially by placing too much tension in the muscles on either side of this lead elbow to fix it in shape, and then make a stroke with that structure and tension remaining the same and the arm pits staying the same throughout the stroke. By getting the lead side alone sorted out, the rear side becomes irrelevant and only supportive.
Backing off a bit to a more relaxed structure, try "rolling" the lead elbow inward towards the chest just a touch in the setup, and then countering this with the lead forearm "rolling" the opposite direction. The first action makes the grip on the putter "weak" with the face a bit "closed" and the second action makes the hand "stronger" back to square with a square face. The combination is similar to "wringing a dish rag" about the elbow joint. This tightens the structure with a special tension up and down the arm thru the elbow, and not just tension. Then pay attention to "where" the inner surface of the elbow aims (it differs for each person slightly -- in my case, the aim is not quite square into the target line but a little "closed" to the right a few degrees). keep this orientation of the inner surface of the elbow the same during the stroke by moving the lead shoulder vertically back. This action will maintain the lead armpit in the same configuration and the inside of the lead upper arm in the same contact with the side of the chest throughout the stroke. At this point, it's just a question of the fullness of the stroke and its fluidity.
Try that. I hope it helps.
Cheers!
Geoff Mangum
Putting Coach and Theorist
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