On the rise

To advance out of this level you must train yourself to predict the bounce of the upcoming shot. You must learn to move your feet toward the bounce. As a beginner/intermediate player your racket can be back and waiting before the ball bounces. As an advanced player, many times, your racket must be moving forward before the bounce. This is called "hitting on the rise" or "trapping the ball." After practicing this approach to the game you will quickly see that the height and pace of the ball will become secondary and you moving toward the bounce of the ball will become primary. You will eliminate most of the errors you have been making by allowing the ball to climb over your eye level. The wind will not be such a negative effect because you will not allow the ball to get up high where the wind will torment you.

You will not spend your life moving backwards toward the fence. You will be able to generate more power because you will be accepting the pace of your opponents' shots on your racket. It is much easier to hit a ball harder on the rise than on the fall. This is why you see the pros constantly hitting so hard. They are using their opponent's pace to add to their shot. Plus, anytime you are moving toward a ball to take it on the rise, you will always have an option to continue going to the net to gain an offensive position. Something you will never do when you are way behind the baseline.

From http://www.tennisserver.com/Mills-tip/Mills-tip.html