PGA Professional Stuart Cartwright from Good Golf Coaching reveals how over-rotating the backswing destroys strike consistency and power. He introduces the '90-degree rule'—maintaining a consistent relationship between the club grip and the chest—and demonstrates specific drills to keep the arms from over-running the body.
Maintain a 90° relationship between the club grip and your shoulder line from address to the top of the swing.
Correct trail hip rotation is the engine that allows the back to open up without the arms over-running.
In the downswing, the feel should be that the grip 'outruns' the chest to prevent the hands from getting stuck behind the body.
Avoid 'throwing' the club early by keeping the hands in front of the trail shoulder during the transition.
Structure at the top of the swing directly dictates the quality of the 'swing moment' through impact.