Golf instructors often highlight the change of direction in the golf swing from backswing to downswing as the one spot where tour pro swings really differ from most amateur swings. If you watched this transition in slow motion, you would see most pros initiating the downswing with their lower body while they are still completing the backswing with their upper body. This "magic move" is key to synchronicity, rhythm and power.
So how do you ingrain this change-of-direction move into your swing? Many golf instructors will advise you to focus on letting your lead hip shift toward the target just as you're completing the backswing. It's good instruction, but without the necessary muscle function just south of the pelvis, it's going to be extremely difficult to do. There are many muscles responsible for this shift, but the ones you should focus on the most are found on the inside of your thighs. They are called the adductors, and are not to be confused with the muscles on the outside of the thighs known as the abductors.
The adductors are really the key muscles to control any type of lateral movement in the golf swing. Not only will they stop you from swaying away from the target too much as you take the club back—which makes solid contact a real challenge—they also are the ignition for that whip-like action that you see tour pros get with their club through impact. In these photos of Rickie Fowler (below), you'll notice a subtle shift of his lower body as he starts down with the club. This occurs when the adductors fire in his lead leg. Try it yourself. Wherever you are right now, get into your golf posture and mimic a backswing. When you are at the top, literally think about squeezing/clenching the muscles on the inside of your lead thigh. If you feel it contract, you've activated it and are now in position to let it help the lower body shift slightly toward the target. This is the power move you've been hoping for. To recap: 1. Take the club to the top of the backswing. 2. Clench the adductor muscles. 3. Swing down.
If you had trouble activating the adductors, Golf Digest fitness advisor Ben Shear has a couple of exercises that can help. Click on the video below to see them.