Having a full shoulder turn is the main source of power in the golf swing. Turning at least 90 degrees and getting your right shoulder several inches behind the ball is one of the fundamentals of a good golf swing.
To eliminate much of the confusion on how to do this, simple think of the word, 'handshake'.
Pretend you're standing at address over the golf ball but without a golf club. Now turn your body, shoulders and torso back and simulate the act of shaking someone's hand. The photo shows what a simple move this is yet how well it turns the shoulders. For your downswing, the left hand will swing through the ball and shake hands on the follow through. Thinking plane, path, inside or outside is way to confusing I think you'll agree.
