The most important power angle in the swing is the angle between your left forearm and the clubshaft. Too many golfers lose this angle early in the downswing–a move known as “throwing the clubhead away”–and this causes the clubhead to actually lose speed before it reaches the ball.
To train yourself to hold this power angle longer on the downswing, grab an ordinary hand towel and wrap it around your driver shaft. Grip the club in your left hand as you normally would and use your right hand to hold onto the end of the towel.
Make some half swings, stopping at the pre-impact position; hold the towel tightly enough so that you can maintain the hinge in your left wrist and the angle between your forearm and shaft. Repeat these half swings until you can grasp what clubhead lag feels like, as well as the way your body must move to create that power angle.
Now remove the towel and hit some drives with both hands on the club. You should notice a big increase in speed and power as you release all that stored energy into the back of the ball.