Advantages: A lot more potential clubhead speed. Most people can cock their left wrist in a vertical plane a lot more (bigger range of motion) with the left hand pre-turned on the grip. Take a 4-knuckle grip with the left hand, cock the club straight up in the air, the left wrist will not be flat, but the left arm flying wedge will still be intact. Think Fred Couples. Do the same with a Corey Pavin one-knuckle grip, and you will see that you lose some range of motion.
"If Freddy Couples had a flat left wrist at the top of his swing, Freddy wouldn't be Freddy." - Rick Smith 1993
A strong left hand grip is suitable for the double cocked positioning of the wrist. You will never see a weak left hand grip and a cupped left wrist on tour.
Disadvantage: You lose the visual equivalent of: flat left wrist = clubface.
Another disadvantage (from Johnny Miller): With a strong left hand grip, the clubface can be square to the target with varying amounts of clubshaft lean. Distance control may be more difficult. With a weak left hand grip (one knuckle / flat left wrist), the clubface will be left open if there is too much forward lean, which promotes (for accomplished players anyway) hitting the ball with the same loft every time, better for distance control.
Imagine you were throwing a javelin with your left arm. Notice the position of the left wrist? Bent back, but still in line with the left forearm.
That is the strongest position of the left arm flying wedge, from a power perspective (although as noted, not as easy to monitor from a geometry perspective).
It is indeed the reason Couples is so powerful.
Shaft lean can be managed, but it requires an understanding, and maintaining, of the bent right wrist angle. Couples and Daly both let that right wrist angle go on the full swing (but notice Daly in his short game, one of the reasons he is so good around the greens).
The turned left wrist is 'insurance' against throw away at the extreme (see Alex Morrison), since it is physically impossible to let the club pass the left arm with it turned so much.
Properly understood, I think a lot of people can benefit from the advantages in a turned left wrist because of this 'insurance'. See Morgan Pressel's action for a good starting pattern for many IMO.
Agreed that Miller's left hand grip is better for distance control, because the weaker left hand is a limiting factor in shaft lean - but - it requires actively 'doing' something to keep the rest of the angles in place. One of the reasons he isn't playing IMO, and one of the reasons Tiger hits wild shots with his current pattern - they are fighting the alignments of the flying wedges -
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