[Yoda]

. . . while the left arm/clubshaft is traveling along/parallel to the inclined plane. Is that correct?

It is useful to think of the Clubshaft as always being On Plane and Tracing the Plane's Base Line). Thus, when the Clubshaft is parallel to the ground, it also is parallel to the Base Line. Otherwise, one end of the Clubshaft or the other is 'pointing at' (not covering) the Baseline.

However, TGM students know that, normally, only the orbiting Sweetspot defines the Plane (2-F). Since the Clubshaft is set an inch or so inward of the Sweetspot, the visible Clubshaft cannot not define this invisible Sweetspot Plane. Hence, in the Backstroke, the Clubshaft must rotate from its own Plane to that of the Sweetspot. Then, in the Downstroke (specifically, in Release), it rotates back to its own Plane. Finally, at the end of the Follow-Through and into the Finish, it rotates again to the Sweetspot Plane.

The Left Arm, except with Zero #3 Accumulator Angle, is never On Plane. Nevertheless, as with the Turning Shoulder Plane (10-6-D), the Left Arm can move parallel to the Plane Line.