Practicing in my basement Lab I have come to realize the value of left arm only swings. TGM imperatives/essentials are magnified.
Here is what I have discovered swinging with the left arm only:
Flat left wrist - you cannot keep the sweet spot on plane without it or maintain the flail.
Law of the Flail - again inconsistent contact without it. The ball will/can go anywhere/nowhere if you break the law of the flail. No left arm chicken wings allowed (sorry 12 Piece).
Left hand clubface - Directional control.
Plane - single plane swing on the proper angle.
Arc of approach - contact the inside of the ball.
Hinging/swivel - the difference between hinge and swivel and how the swivel happens after hinge and after contact.
Stationary Head - Any bobbing and you are finished. Swaying tough to get back to the ball and make consistent contact.
Which leads me to surmise that the left hand/wrist/arm (hinge) is the heart and soul of the golf swing. Be the one armed Iron Byron machine.