Another great tool you can use if you are looking to improve the length of your back swing is the redundant back swing drill. Now this again is not an easy one to consistently hit shots with however what he does do is isolate certain parts of the swing so you can repeat you can practice and you can get the feeling of certainly the top of the back swing moving into the downswing.
Longer Swing Golf Drills - The Redundant Backswing Drill (Video) - Lesson by PGA Pro Peter Finch