It's not uncommon for amateur golfers to incorporate little or no shoulder turn in their swings. Some actually might believe they're getting shoulder rotation when the reality of their swing is something very different. Although the idea of adding shoulder turn to your swing might be daunting, there are several drills that can help simplify the process and bring this source of power to your game.
The coil created by upper body rotation in the backswing operates as one of the main sources of power in hitting golf shots. Power is not generated by the arms, as believed by many. A proper shoulder turn also helps put the club in the proper position at the top of the backswing, increasing the chance of coming into impact on the right swing plane.
Shawn Humphries describes learning about teaching shoulder turn from legendary golfer Byron Nelson. Nelson suggested that one way for golfers to gauge their true shoulder turn is to keep the hands directly in front of the sternum through the backswing. Doing this prevents the arms from traveling back any farther than the shoulders. This might feel a bit awkward at first, but begin by swinging slowly and you should soon find the shoulder turn becoming easier.
Renowned golf instructor David Leadbetter suggests a slightly different relationship between body parts for monitoring a full shoulder turn. Leadbetter recommends that you seek to bring your left shoulder under your chin at the top of the backswing. Doing so means that not only have you rotated your shoulders fully, but also that the orientation of your head remains in its proper place.
Dan Frost of Today's Golfer makes reference to the role of the lower body in gaining full shoulder rotation. He suggests keeping your right knee firm throughout the backswing while focusing on turning your core, as opposed to your shoulders. Turning the core of your body, of course, also will rotate the shoulders, but will likely be an easier task for your brain to execute.