Do you think the term "plane shift" is apt when what most /all people do ( including Leadbetter - hence your X classification) is a "plane drift"?
Plane angle can readily be defined at 4 points in all swings.
1. Address
2. Left arm horizontal to ground
3. Top/end of backstroke
4. Impact
plus maybe Follow through/ both arms straight.
Most golfers ( I would argue all golfers) drift planes between these positions. During drift the butt/end of club rarely points at target line unless the drift happens rapidly around clubshaft horizontal ( and therefore ideally parallel to plane line).
Leadbetter people basically are double shifters - categorised by their address / impact location and top/end of backstroke positions. They may drift a bit more than others off TGM plane at left arm horizontal but 90 % of people drift not shift!
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