Fundamental 3 Use of the left arm
Each player has their own swing and the shape of this swing is largely created by the use of the left arm. The movement of the left arm has both an around and upward directions. Swinging the left arm around your sternum combines the around and upward into one smooth movement. The sternum is of course your swing centre.
By adding the shoulders with the left arm the conceptual shape changes from a single spoke to that of a number 7. You can now see the shoulders as a wheel and the left arm as a spoke. Note that the shape is a 7 and not L. The shoulder part of the 7 should not wobble about but move on a transverse plane between the neck and the ball. Timing is achieved by maintaining this 7 shape throughout the swing.
Fundamental to the downswing understand that the left arm is advanced downward from the top of swing to impact by weight transference alone. This means that the left arm is not dropped, not pulled and not harpooned back in the downswing.