It is how the bending and straightening of the right elbow raises and lowers the left arm and at the same time cocks or uncocks the left flat wrist without disturbing the bend flat right wrist. The right elbow action of folding on the up stroke and straighten on the downstroke allows the right forearm to still On Plane - an incline plane with three dimensions and a circle. A great place to be.
I'm sure a better answer awaits you.
