From my TGM notes on Brian and Yoda's posts:
Now to your question and how to determine the exact amount of waist bend:
1. Get your Driver.
2. Standing about Driver distance from the Ball, think of the Driver as a hunting rifle. Put the Clubhead into your Right Shoulder and hold the Club just as you would aim a rifle. Get the Feel of the Rotated Shoulder Turn at first just by keeping your rifle barrel level with the ground, and turn from left to right and back again, over and over, getting the Feel of the perfectly level shoulder rotation. Pretend you're in a carnival shooting gallery and you are knocking off ducks at various points on the never-ending line of the conveyer belt. Make sure you are taking 'dead aim' by sighting directly down the 'rifle barrel' using your right eye.
3. Once you've got this motion down pat -- should take less than a minute! -- we need to move off the horizontal plane and onto the exact Rotated Shoulder Plane. So...
Keeping the 'butt' of your 'rifle' into your right shoulder -- and not dropping your arms in the slightest. Careful, this is the tendency! The 'rifle' must remain in the same relation with the Right Shoulder as it did in the horizontal drill above -- bend forward from the waist until you are 'aiming' your rifle (the butt-end of the Club) directly at the Plane Line.
This is the exact amount of waist bend you need to accomodate the On Plane Rotated Shoulder Turn in both directions.
Taking this a little farther, after you've located the exact waist bend, get the Feel of the On Plane Rotated Shoulder Turn motion by rotating the shoulders (at right angles to the spine) the same way you did when you were 'aiming' in the horizontal plane, but this time 'aim the rifle' at the Plane line and 'Trace' it in both directions of the turn.
Summarizing:
1. To determine the exact waist bend, 'shoot' the Plane Line with your 'Right Shoulder rifle.'
2. To maintain the Right Shoulder on its Rotated Shoulder Plane, 'Trace' the Plane Line with your 'rifle barrel!'
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Yoda