[EdZ]

The first part of the physics is lag, the 'cracking of the whip' from the ground up. At the start of the downswing, the 'whip motion' moves through the body, to the club, starting with the left foot, left knee, left hip, left side, left shoulder, arm, wrists, club. You can push with the right in 'feel', but pull or push, you must maintain the hands leading, or you have 'lost the whip'. At impact the 'bend in the whip' is the arch of your lead wrist, see Hogan p 102.

If we somehow could magically have a completely verticle 'whip', then no rotation would be needed (well actually, just a 'bit' because of the design of the club) - if it were more like a croquet mallet, you could though. The fact that there is an angle between the shaft and the clubhead means that physics will tend to lead to the toe rotating to be in line with the shafts motion/plane.

So we have the first two parts, the physics of lag, and the design of the club.

The third is the body. The club follows the hands, which follow the arms, which follow the shoulders, so where the shoulders move is one of the more critical components. Feel like you 'hit the ball with the back of the lead shoulder' (or for some, the front of the trail shoulder), will let you feel the 'triangle - that the entire shoulder line and arm span is one big unit/triangle - its 'tip' is where the hands meet, and that is the 'point' that both hands are either pulling, and/or pushing - the last 'hinge' in the whip motion, of the flail.

If you take your grip with your palms square to each other, there is a conflict between how your left hand will align when swung on its own (stronger, butt of hand leading) and how your right will align on its own (more 'square', palm facing planeline)

So if you have that grip, you will have to have rotation in the lead arm but standing on the side of the ball, and the physics of the club design, but contribute to the left hand 'rolling' back to square IF you let them (relaxed) or force them (intentional roll).

If you don't, you're going right OR breaking the 'whip' to square up.

I would encourage most people to look at Alex Morrison's approach to the grip, because it accounts for the natural motion of the hands and arms 'in the whip'. It does not fight the natural move of the left arm, nor does it 'roll', until well after impact if at all.

My preference, as stated in the first post, is to have the lead hand at '45 degrees', which allows for a more 'direct line' takeaway, a good way to feel right forearm pickup.