The role of the backswing is to put you in a proper position to deliver a powerful downswing. When you pull the club away, the muscles become stretched. They will now contract more forcefully on the downswing.
When a club is pulled up and back during a backswing, it produces a pathway over which you can generate maximum club head speed. The length of this pathway is determined by your stomach and shoulder flexibility. The club will also be a factor.
Rotation and twisting type exercises will be very effective in building up your backswing. There are many good books out on this subject.
