There are two things that come into play that give power to your shot. One is momentum and the other is acceleration. Momentum is the force from the forward speed of the racquet, while you can increase that force considerably by accelerating the racquet.
I have been de-emphasizing momentum so people pay more attention to acceleration.
Why? This is because minimizing the backswing, or taking the racquet slowly up to the ball (or close enough), helps find it and control the shot. That usually corrects unforced errors.
Now, when you become a master at this and your attention stays in front and not behind you, you can make your swing bigger and bigger.
What I usually say to a student, after he or she masters the control of the ball, is to hit the ball harder, then harder, but GRADUALLY, so they take the racquet back without THINKING of the backswing. As a result, the backswing becomes bigger, but the player is not thinking about it. His whole attention stays in front and then on the finish of the stroke.
If the person has learned a good finish across the body, this action of delaying the backswing actually increases the finish.
Witness the story of Andre Agassi Wimbledon win in 1992. Before Wimbledon Agassi was practicing with John McEnroe in Paris. Agassi asked McEnroe: “how do you play on grass”, to which McEnroe replied: “on grass courts, no backswing”.
Does this mean that Andre Agassi did not have a backswing at Wimbledon? Not quite, but his attention to keeping the racquet in front delayed his backswing to where his whole attention became instinctive, practically instantaneous, and he Zoned in (seeing the ball slower than usual). Which prompted, in later years, Agassi’s seemingly illogical statement: “I hit the ball when it stops”.
Thank you, John McEnroe, for this tip. I consider it the greatest tennis tip ever. Sorry it cost you your 1992 Wimbledon (McEnroe lost to an inspired Agassi in the semifinals, in what was deemed to be, by that stage of the tournament, McEnroe’s year to win).
Why is this process so successful in teaching? Because your backswing becomes instinctive, produced by your desire to power the ball, not a product of stuck pictures in your mind!