The crux of the reasoning can be found in chapter two. The line of compression.

As mentioned, 'the' plane is not the clubshaft, but the line of force between the hands and sweetspot.

that is a very critical part of TGM to understand.

For the physics of chapter 2 to create maximum compression, plane shifts should be kept to a minimum, because any shift is likely to decrease the force and efficiency of the line of compression (that said, there are more advanced discussions to be had around shifts and power generation, suffice to say from release point through both arms straight, any shift is not efficient).

As far as the body's role, keep in mind that it is the hands that must retain the relationship with the plane line, the body must just comply (see Ted's clip of his wacky pivot 'hit' in the gallery)

In a nutshell, the body must move in a way as to

a) allow the hands to retain the relationship to the plane line and

b) get the right shoulder moving down plane with as much of the body's mass as possible supporting impact while maintaining a stable center

Actually, earth gravity could make a difference here. It is constantly pulling the club towards the ground and may support a gradual plane shift - or actually a spiral-like plane where an uncompensated single plane stroke is combined with a constant, vertical force.

It would be interesting to sort out whether the educated hands know where the ball is in gravity-less conditions. Surely there must be a golfing astronaut who could find out for us?

Gee - this plane stuff seems really complicated when you dive into the details.

I thought the LOC was vital throught impact only. In that case, as long as the SS-plane is aimed at the impact point from release to follow though (between 3-4th parallels) proper LOC should follow, no matter the SS-planes orientation before reaching 3rd parallel. Keeping planeshifting to a minimun is obvious, but wether this is the case in any swing depends on the definition of "on plane" in the first place.
That for me is the crux, the fundamenals behind the "chosen" definition of being on plane are still somewhat unclear to me (from a geometric/physics view that is). My feel/instinct 'get it' but the mind is lagging behind. :BangHead: