Concept
There is a constant relationship between the arms, shoulders and the centre of the chest through the swing.
The picture of this concept starts with the shape of a number 7. Your shoulders represent the top-line and the left arm is the downward stroke of the 7. The left arm stays in its position relative to the shoulders throughout the swing. At address this 7 shape is over the centre of your chest. Your goal is to have the upper chest and arms working together. In this way the arms swing around the sternum as the trunk rotates.