Trace the backswing arc with your eyes and ears
To lengthen the backswing, trace the swing's arc with your eyes or ears. Let the lead ear point toward the ground and the trail ear point upward so the head can tilt and the shoulders can continue turning; the instructor also describes this as pouring water out of the lead ear.
Channel: Danny Maude
Watch official source at 4:20Why this context matters
Instructor stresses this especially for senior/less-mobile golfers, though he applies it to all.