Is turning the knob just done with the left arm? Care to explain how this is done and why you do this ...thanks
If you put the club straight out in front of you and think about how you would twist the clubface open or closed, the only two parts of your body that do this twist are your upper and lower arm. You'll also notice that you can twist your forearm independently of your upper arm. I think of this independent twist as "turning the doorknob" since when you turn a doorknob you only do it with your forearm.
When Brian started discussing his tumble idea, it clicked even more for me. "Tumble" is essentially the upper arm twisting.
But someting else that happens with this upper arm twist, is path control. The more counter-rotated the upper arm is, the more "from the inside-out" you swing the club. The more rotated the upper arm is, the more "from the outside-in" you swing the club. But since tumble also affects clubface, you have to offset this with forearm counter rotation and rotation.
So, learning how to control both upper arm and lower arm rotation is a way to control both path and clubface. It allows you to hit draws and fades on demand as well as identify how to quickly fix problems in path or clubface. Once you have that nailed down, you can adjust your stance and ball position to hit any shot you want.
You can tumble and turn the knob with either arm since both arms are attached to the club and affect each other. I personally like to monitor my left arm because I've also learned to control my kinetic sequence with the use of my left arm (whole other story).