[Yoda]

Lynn, does that description of yours above (post #13) also provide an accurate depiction of why the shaft does indeed rotate around the sweetspot?

Can it (and/or SHOULD it) be explained through plane angle? That is, the clubshaft must shift to the sweetspot plane going back and once again after impact?

As components of the Left Arm Flying Wedge, the Flat Left Wrist, the Clubshaft and the Sweetspot must all remain in the same plane, i.e., the plane of the perpendicular Left Wristcock motion. This sounds complicated, but it is not. For example, when hammering a nail, it is easy to see that the left hand and the hammer handle and the hammer head all lie in the same vertical plane.

On the Backstroke, the entire Left Arm Flying Wedge Turns to the Sweetspot's Inclined Plane of Motion (2-N-0) and, on the Downstroke, Rolls from it. First, the Left Wrist rotates (Turns) from its Vertical (to the ground) Condition to parallel to the Sweetspot Plane. As it does, the Shaft must likewise rotate in order to maintain its In Line Condition with the Left Arm (Rhythm per 6-B-3-0). Similarly, on the Downstroke, as the Left Wrist rotates (Rolls) back to its Vertical Condition, the Shaft must also rotate.

Otherwise, the Left Arm Flying Wedge -- the In Line Condition of the Left Wrist, Clubshaft and Sweetspot -- cannot be maintained. Then, there is Steering and Quitting.The Left Wrist Bends; Rhythm is disrupted; and the Line of Compression can no longer be sustained.