People ask me all the time what stops the backswing. Or what starts the downswing.
The answer is the same: At the change of direction, it's the separation between the lower body and upper body. We'll assume you've set up well, kept your grip light and made a full turn so you feel tension in the left side of your back (above, left). Think of the old image of turning in a barrel to prevent sliding off the ball.
The lower body starts forward while the upper body is still going back: The left hip starts to turn toward the target as the shoulders continue to coil. This separation creates torque that has to be released later, resulting in arm speed through the ball. Bobby Wilson, the two-time senior long-drive champion, told me that more arm speed is the only way to add distance. Separation between the lower body and upper body (above, right) is the key in any sport where you use the hips to create arm speed -- think tennis serve or baseball swing. Ted Williams talked about it, and so did Ben Hogan. Golfers need to think about the lower body leading the downswing.
NO LIFTING (above, left): I feel as if my left shoulder stays down as my hands and arms drop.
RELEASE (above, right): As long as your lower body is leading, you can throw your right hand hard.
With the left hip starting the downswing (the left heel replanting if it came up), keep your shoulders turning 90 degrees to your spine. I almost feel my left shoulder going down and around (above, left). That's a point made by David Leadbetter in his teaching. Byron Nelson talked about feeling "leisurely" at the top. Let your arms drop quietly, then swing them through as fast as your body will allow and stay in balance. Throw the club through the ball with your right hand (above, right). As Jack Nicklaus said, you can hit as hard as you want with the right hand as long as the lower body is leading.
To review: The sequence from the top is, left hip reversing direction, left shoulder down, right hand releasing the clubhead. Fred Couples makes practice swings with only his right hand on the club, and he has wonderful rhythm and power.