I can understand the concept of a sweetspot inclined plane being different to the clubshaft hosel inclined plane, and how the angular difference is at its maximum near the low point of the swing, and at its minimum at the end-backswing and finish positions. However, the angular difference is very small in practical terms, and presumably it is sufficiently accurate to roughly trace the SPL with the right forearm/PP3. Prior to reading the TGM book, I used to use lay-back when hitting out of bunkers or hitting flop shots. However, I simultaneously allowed my arms to steer towards the target while using a vertical hinging action, and my left arm could be closer to the ball-target line than the inclined plane. My question is whether it is very important that I should deliberately learn to ensure that I trace a SPL in the followthrough phase of the swing while using vertical hinging?
Not sure if this is right but I think that you are asking about the difference between dual vertical hinge ( which keeps the shaft/sweetspot on plane) or the vertical hinge which doesn't...
You are describing a controlled steering procedure i think... for non-full power shots it probably has little bad effect...as long as you know what you are really doing and not what you think you are doing... then you can repeat it and modify it according to need.