3.? Spring toward the ball, keeping your head and hitting elbow up
The main thing that people do wrong after they execute a good toss and knee bend is they crouch down while they swing. It looks as if they're folding over at the waist. This is problematic for putting pace and spin on the ball. You must extend and reach up to use your full force and get power on the serve. The upward thrust is key for completely straightening the back and beginning the uncoiling of the upper body.
After the racket head drops down, the hitting elbow should be at least at shoulder level. You can achieve this by keeping your head up and holding your tossing arm in position a split-second longer than usual. When a player pulls his head down, or the tossing arm starts to drift and drop too soon, the upper body often comes with it. Keeping your head and tossing arm up while straightening the legs will get you in the best position to allow your racket to whip through the hitting zone.
4.? Brush up the back of the ball with as much racket-head speed as a first serve
When players first learn the kick serve, the concept of swinging up on the ball is tricky. Most feel they're going to hit it over the fence, so they try to spin the ball toward the box instead of up and generally hit a very short serve or one that doesn't clear the net.
One way to get the sensation of brushing up the back of the ball is to put some towels on the court and serve from your knees. From this position you have to swing up in order to clear the net (and not smack your racket into the court). Concentrate on trying to get the ball up 6 to 10 feet over the net and allow the spin to bring it down into the box. Using a continental grip (even slightly toward a backhand grip) is strongly recommended.
The wrist should also play a huge role in the racket accelerating up the back of the ball. You can practice the snapping motion of the wrist without holding a racket. Simply hold the ball in your off hand and brush up the back of it with the palm of your hitting hand to get comfortable with the wrist action. The other thing to keep in mind with your swing is to let it go. So many people, because it's a second serve, decelerate the racket. You have to trust that the heavy topspin you're putting on the ball will keep it in the court.
5. Uncoil at the top of your swing and follow through
After you reach up to hit the serve, your upper body should uncoil toward your target. When you can't reach any higher, the racket is naturally going to go forward. You don't want to uncoil too soon by bringing your back leg around because you would prematurely face the net. This would cause you to fall to your left (right for lefties) and drain your shot of power and spin, not to mention leave you off-balance and out of position for the next shot. Try to keep your arm loose and allow it to finish naturally and help prepare you for your opponent's return.