[ChrisNZ]

Let's say you have a shift from the elbow plane to the turned shoulder plane on the backswing. You haven't bent the flat inflexible inclined plane; there are, in fact, two: the flat inflexible elbow plane, and the flat inflexible turned shoulder plane. You simply moved from one to another. This is my understanding of the concept.

Bigwill,

I basically agree, and its a minor point, but I prefer to think of one flat inflexible inclined plane, that is basically hinged on the ground (exactly where it is hinged is a little more complex, but I won't go into that!). A plane shift involves moving the whole plane around this hinge. Then when you're shifting planes your not 'between planes', but in the process of 'moving the plane'. Kind of like a door - it can be at a basically limitless number of positions in relation to its hinges (all different 'planes') but it can move from one position to another while continuing to point at it's hinges.
Chris