It all starts with a loop backswing. Turn your shoulders and bring the racquet back between shoulder and eye level. On the one-handed backhand and forehand, you can use your non-dominant hand to lift the racquet above your hitting wrist and facilitate the shoulder turn. If the racquet is going to pause anywhere in the swing to wait for the incoming ball, it should be at this point. Because as soon as the racquet drops, the swing should accelerate smoothly through the hitting zone.
Another point of controversy among players, coaches, and tennis professionals has been which type of backswing provides more racket velocity and control. It was thought that the traditional straight backswing provided more control, and the loop (large and small) backswings provided greater racket velocity. Although a large-loop backswing has been shown to increase racket velocity, racket control and timing are more likely to be affected.. In contrast, the small-loop backswing seemed to increase racket velocity without affecting the timing and control of the stroke. Regardless of the type of backswing used, for more power and efficiency, the transition between the backswing and forward swing should be a fluid motion since it enhances the player's ability to utilize the pre-stretching of the muscles.
In tennis, power starts from the ground up. Just like a boxer throwing a punch, you want your legs, hips, and trunk involved in the swing, not just your arm. So as the racquet goes back and the shoulders turn, you should coil your hips and trunk and sink down a little on the back leg. Now all your energy is loaded up and ready to release into the shot.
As the ball approaches, drop your racquet one to two feet below the level of the ball and make sure that the face is closed (almost parallel to the ground). This will allow you to hit up on the ball and use your wrist to give it topspin. As the swing starts forward, the uncoiling begins. The legs straighten slightly, the hips and trunk rotate toward the ball, and your weight comes forward, bringing the racquet along for the ride. Because your hitting shoulder is behind you on the forehand, it's particularly important to get good rotation in order to make contact out in front of your body. As you swing, pretend the ball is the face of a clock--to apply topspin to it, accelerate the racquet from 6 to 12 o'clock and extend through the hitting zone.
It's common for players to ease up on their swing or to try to come over the top of the ball. They fear such an upward motion will cause the ball to fly, so they compensate. In reality, on a topspin shot you've got to swing even harder than you would on a flat drive. If you brush up the back of the ball but don't come through the shot, you'll get the spin you want, but you won't get any pace or penetration. The goal is to get what I called "Pop Top." This is a topspin shot that's got some juice behind it because you've swung with force and extended fully through the hitting zone. To properly finish, allow your racquet to slow down naturally on the follow-through. If you have an Eastern to semi-Western grip, your racquet will probably finish in the area of your opposite shoulder. If you use a Western grip on your forehand, the racquet will more likely end up around your opposite hip.