The left shoulder sets up the backswing by rotating down and under the chin. This shoulder movement directs the clubhead travel to the inside of the target line. The shoulder turn determines the length of the backswing.
When starting the downswing the shoulders should be passive, allowing the forward motion of the legs to pull the right shoulder down and forward, squaring the shoulders to the target line at impact. If the shoulders have moved into an open position (facing to the left of the target) prior to impact, the clubhead will travel outside-to-inside of the target line. This brings the clubhead over the ball promoting an over the top move.
If the shoulders are in a closed position (facing to the right of
the target) the clubhead travel will be inside-to-outside of the target
line. With the shoulders parallel to the target line at impact, the
clubhead travel will be inside-to-direct. This shoulder motion allows
the clubhead to travel to the target, not away from the target as in
the other positions.Remember the shoulders must remain passive, and
are a follower in your swing, not a leader.