Pronation is the action of moving your thumb in towards the center of your body.
When you are serving, your racket hand should be approaching the ball with the little finger first (if you have the correct grip).
As you go to strike the ball, your hand needs to change direction by moving round so that the thumb moves towards the ball -- just like you were going to do a high five.
At the pro level, you will see the butt of the racket (bottom of the handle) aimed at the ball when the player is in their pre-throw (or trophy) position and ending up with the butt pointing up to the sky after the player has fully pronated their hand.
When this is done at high speed it has the same effect as the movement that causes a whip to "crack."
A great way to practise this action is to stand on the service line facing the baseline with a tennis racket and a ball. Drop the ball over to the side of you that has the racket and after one bounce try to hit the ball behind you and over the net.
To do this effectively, you must pronate the hand and wrist in the desired fashion. The better you become at it, the higher and further you will be able to hit the ball behind you.
60 percent of the game of tennis is actually serving and returning. But does your practice time reflect this?
If you are like most people, you will spend most of your time hitting forehands and backhands and then some volleys.
After that, you may hit a few serves to warm up, and then you play some points. But how much time do you spend actually practicing your serve?
Developing a big first serve and an accurate serve is all about getting on the court and practicing with power and putting targets down to develop accuracy. A big part of your success in developing a consistent, reliable and powerful serve will depend on the time spent with nothing more than a basket of balls and your desire to serve as many balls as you can.
Try adding one of these components into your routine for a week before moving on to the next step. Before you know it you will start to see your opponents moving further and further back in an effort to try and deal with your delivery.