There are two phases to the golf swing, the backswing and the downswing, both hold equal importance.
Talking specifically about the backswing there are some key movements that if achieved will result in power and consistency. Figure 1 shows the set up position.
The first movement of the backswing should be initiated with the shoulders without using the hands to change the angle of the club face (figure 2). There are many tips written on how to achieve width on the backswing, as this will provide more power. This is often taken the wrong way and golfers will end up pushing the club head out very wide without any hinging of the wrists.
Width should only be achieved by keeping the left arm fully extended. If the arm bends at the elbow joint, which is very common, then the width will be lost along with the power. At the same time that the left arm is being kept straight, the wrists should be hinged to 90 degrees just as the left arm reaches parallel with the floor (figure 3).
Make no mistake, it is a difficult move to hinge the wrists whilst keeping the arm extended but with practise swinging in front of a mirror it can be achieved. When the wrists are 'set' in this position, the only other move that should be made is to turn the shoulders further, taking the club to the top and ensuring that this 90 degree angle is retained (figure 4).
If you can keep your width with your arm, hinge the wrists and complete the turn with your body you will have satisfied all the requirements of a poised and powerful backswing.