[Yoda]

Does that mean for the Hitter, because he gradually Turns his Left Wrist to an On Plane position at the Top, with the motion of his Arms and Pivot (as opposed with the Forearm itself), that the Cocking of the Left Wrist will cause the Clubshaft to be Off Plane in the beginning of the Stroke?

Any action that produces an awkward or 'un-golflike' motion is improper execution (3-F-6). And arriving at the Top of the Stroke with the Left Wrist vertical to the Plane as johngolf33 described produces exactly that in spades. It just 'ain't natur'l.'

On the other hand, the move of the Hitter is 'natural' defined. The Wrist Turns on the Backstroke in exact concert with the Body Turn and Arm Swing. As Extensor Action causes the Right Elbow to bend, the Left Wrist Cocks just as it should. Through it all, the Clubshaft remains On Plane with Club pointing at the Line except when they are parallel (1-L #6).