Many weekend golfers can rotate their shoulders 90 degrees or more on the backswing, but they often do so without turning their spine. In instruction parlance, this is often referred to as a "fake turn," although it could just as easily be described as a "weak turn" — it's hard to generate much coil or power if your shoulders are doing all the work. You need to turn your entire upper body — which includes your spine — behind the ball if you want to create as much potential energy in your backswing as possible.
SET THE "T" AT ADDRESS
To make the best backswing turn possible, set up with your spine tilted away from the target and your right shoulder lower than your left, so that your shoulders and spine form a tilted letter "T." As you swing back, think about turning the entire T, not just your shoulders, as one unit. This will get you in the habit of rotating your entire upper torso, spine and all. That's how you generate a lot of coil.
MAINTAIN THE "T" INTO THE FINISH
You're not done yet, however. To ensure that you take full advantage of the momentum you created on the backswing, you need to finish your swing with the T in place. In fact, as the illustration above shows, the right shoulder should rotate through until it points at the target. This keeps the club on plane — i.e., traveling at a right angle to your spine — and moving at maximum speed through impact.