Good backstroke requires a certain amount of rotation, but this rotation also has benefits in reducing the resistance you create recovering your arms.
Why do it:
Focusing on leading the recovery of backstroke with the shoulder can simplify the thought process concerning rotation. Focusing on something specific, can cause something to happen more naturally.
How to do it:
1) Start with both arms down by your side and push off on your back while kicking flutter kick.
2) Rotate one shoulder up and recovery the arm like you normally would.
3) Take a stroke with that arm, and as the hand enters the water, pop, or rotate the opposite shoulder above the water and place it close to your cheek.
How to do it really well (the fine points):
Try to hold that shoulder up through much of the pull, which should cause the body to snap back more quickly. Building this connection between the body and the pull will allow for a stronger connection with the water. This drill also has the benefit of reducing the amount of resistance that occurs during the recovery by getting the swimmer used to popping the shoulder above the surface of the water.