[Scottgas2]

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One basic reason would be, all things being equal, a steeper shoulder plane would leave the face open, while a flatter shoulder plane would close the face prematurely.

Now there's an AhHah moment. The flatter shoulder turn would thereby promote hooking the ball? I have noticed, several instructors stating that lowering the right shoulder is necessary to tilt the hips improving the pivot. I think they are promoting a vertical drop of the right shoulder before the pivot rather than right shoulder travel down a plane to the ball. This is a bit confusing.