[nevermind]

Technically, using ANY plane, whether downstroke or backstroke, if one does not shift from one plane to another, then that means it is a "zero shift" stroke. A zero shift is not limited only to the downstroke.

If on the backstroke I elect to shift from an elbow plane to a turned shoulder plane then that is a shift.

Come on man, I knew that... so back to the question. If you make a shift in the downstroke, which plane is the preselected Downstroke CLubshaft Plane, that the shoulder should move precisely on? The plane your shifting from or to? Or is there no preselected Downstroke Clubshaft Plane if there are shifts in the downstroke?

Sure it can. I merely have to switch planes. In fact, you will see that such shifts are pretty common on Tour. Usually you will see a move from the turned shoulder plane to the elbow plane.

I'm going to assume your talking about the clubshaft here, coz I definitely can't picture the rear shoulder getting down to the elbow plane Should the rear shoulder make plane shifts during the Downstroke? If the rear shoulder is on the Turned Shoulder Plane at startdown, no matter what shoulder turn was used, how can it move downplane at startdown on anything but the Turned Shoulder Plane?

You may not like this answer, but the rear shoulder should move downplane as long as it needs to. As I observe in all my students, there is too much around as opposed to "down".

So how do I know how far I need to keep the shoulder on plane for? If it's an individual thing, I guess trial and error is the way to go? How do I know if my bad shots are due to not keeping the shoulder on plane far enough or something else?