The transition from backswing to downswing is where many golfers lose power. They tend to unwind their upper body too quickly from the top and overuse their arms and shoulders, which counterintuitively slows the clubhead down. To maximize your swing speed and unlock 15 to 20 extra yards, you need to engage your hips more on the downswing. If you can get them to "lead" the arms and shoulders in your downswing, you'll bomb it past your big-hitting pals.
LEAD WITH THE HIPS, NOT THE ARMS
Turn your shoulders as far as you can to the top while maintaining your original address posture. From there, initiate the downswing by turning your belt buckle and hips hard to the left while keeping your back to the target. You should feel as though your hips are leading your upper body, then pulling your arms and shoulders into action. This separation between your upper and lower halves is what drops the clubhead down into the correct "inside" power path, making it easier to swing your arms in front of your body and rotate the clubface to square. The separation also helps you maintain the angle between your lead arm and the clubshaft farther into the downswing, creating an extra burst of speed through impact when the angle straightens.
One big key to power is maintaining the separation between your hips and upper body in the transition from backswing to downswing. As you start down, try to keep your back facing the target as you unwind your hips. Your shoulders, chest, arms, hands and the clubhead will naturally follow.
CHECKPOINT: SHAFT ON RIGHT FOREARM
If the clubhead approaches the ball on the correct inside path, the shaft will appear to bisect the right forearm when your hands are about waist-high on the downswing. Your shoulders will remain closed to the target line (i.e., pointing right of the target), with your hips already several degrees open. The more separation there is between the hips and the shoulders, the more distance the clubhead has to travel and the more energy you can unleash into the ball. Get this hip-first move right and you'll be able to hit the ball hard with confidence.
Check to see that the clubshaft appears to bisect your right forearm as the club reaches parallel in your downswing. This tells you that the club has dropped into the proper inside path.