You wrote-: "We're (me, myself and I) talking about only one of those 6 DOF's. We're talking about ROTATION about the Z axis in our arbitrarily defined golf swing coordinate system."
You might be writing about rotation about the Z axis. I am not - I am writing about how the left hand rotates in space during the golf swing and drags the Z axis with it so that both the left hand and the Z axis rotate as an unit around the instantaneous axis of rotation of the flat left wrist/hand. I do not believe that the left hand is applying a torque force around the Z axis when the flat left wrist/hand moves in space. I think that a torque force around the Z axis only develops if the body stops rotating post-impact and the arms keep moving uncontrollably around the stalled body. The key to a good golf swing is keeping the arms rotating in synchrony with the pivoting/rotating torso so that the flat left wrist/hand rotates slowly in space. If a golfer follows AJ Bonar's recommendation in the next photo, then he may be applying a torque force around the Z axis.
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