Arms & trunk
The left arm stays in its position relative to the shoulders throughout the swing.
The 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. At address this 7 shape is over the centre of your chest. Your goal is to have a constant relationship between the arms, shoulders and the centre of the chest through the swing. In this way the arms swing around the sternum as the trunk rotates.