In order to learn how to get more extension and turn when initiating your backswing, there are a few simple tips that can help you accomplish your goal. The mistake that most players make is loosening up their grip too much, especially with the left hand (on right-handed players).
By all means, loosening your grip will definitely help your clubhead extend further behind your body, but then you must re-tighten the left hand which causes a change in the alignment of your clubhead. The result is far less accuracy when hitting the ball.
It is best to always maintain a firm, yet light grip on the club. Varying your grip pressure during a swing is not recommended. By far, the best way to increase the arc on your backswing is to turn your left shoulder further until it has reached a point over the right foot. This way you will always have a full turn, thus creating the fastest speed of swing that you can possibly generate, while still maintaining balance and grace.
The Top Of Your Swing
At the top of your backswing, always check the position of the club shaft in order to guarantee that you are on the proper plane and accomplishing a full backswing. The club should be perfectly parallel to your target line, and the clubhead should literally be pointing at the target.
An easy way to accomplish the changes needed in your swing is to have a partner look at your moves. Better yet, have him or her videotape your swing so that you can determine the changes needed so that your club will point directly towards the target and your swing will be on the correct plane.
If you are a right-handed golfer then check to see if the club is pointing left of your target. If so, then the club is considered to be "laid off" which typically results in a slice. When the club is pointing right of your target then you have "crossed over" which may result in a hook. With a little work and practice you can fine tune your backswing so that the clubhead will always be pointing directly towards the target.