OK... let's get more specific. Mr. Hoch, for example, in the pictures posted by Annikan, has the butt of his club pointing well inside the baseline. This would be Off-plane according to many. He, however, is an excellent ball striker. Is this an example of a correct variation, for someone on his very STEEP Backswing Plane?
Not off plane in my view - just a very steep plane - and a very steep path\plane of his hands motion (the path of the pressure points in 3 dimensinonal space).
You have to watch what the hands/pressure points do. The plane is defined by the hands (pressure points) and the sweet spot - not the shaft - and unless you have zeroed out #3 - far better to just watch the path of the hands for 'the' plane.
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