The transition from backswing to downswing is crucial to generating power and accuracy. The key is to start the downswing with the lower body.
In the best swings the lower body starts forward while the upper body is still turning back. The left hip turns toward the target as the shoulders continue to coil. That takes terrific timing and a lot of practice.
The average golfer should think more about initiating the downswing by rotating the left hip and simultaneously moving to the left heel--this is more pronounced if you've let the heel come off the ground in the backswing (as I recommend). Flaring out your left foot a bit at address will help your hip turn.
Fred Couples is an excellent example of someone who gets the lower body moving before the upper body. His swing is so smooth it's easy to spot.
Also, when you start down, your left shoulder should stay on the plane you established at address. So many amateurs start the downswing by turning the shoulders toward the target too early, causing the dreaded over-the-top move that results in pulls or slices.
Practice starting the downswing in slow motion to get the feel of the lower body leading and the shoulders staying back.
Like my pro-am partners, I'll tell you to feel lazy at the top.
MORE THOUGHTS FROM TOM
Accelerate the putter through the ball. Practice hitting, say, 10-footers with a short backstroke and longer through-stroke. To help, put a tee a few inches behind the ball. Take the club back without hitting the tee, and then you'll have to accelerate.
Tom Watson is a Golf Digest Playing Editor and the golf professional emeritus at The Greenbrier, in White Sulphur Springs, W.Va.