I’ve heard that many times when i had a chat with the friends of my friends. Compare to others sports, such as athletics, or gymnastics or climbing, in tennis there is a dominant technical part which allows you to some some powerful strokes.
It’s why a 9 years old bye can beat an adult.
First of all : All is in movement (Except the serve)
Put in other words, the ball that you receive has already some energy. The purpose is to able to use the energy from opponent’s.
To able to do that, you need to reduce the length of your preparation in order to release the energy at the right moment. (the timing).
Hold the racket softly will allows you to let the energy flowing until your fingers tips.
If you are to tensed, you will generate tension in your forearm and you will likely to be injured after a while.
Furthermore, holding the racket softly will allows you to do your follow through more naturally and to have a better touch. It’s like drawing.
It will helps you to adjust the shape of your movement and to find your natural swing.
The energy comes mainly from the ground
As a basket ball player, he is able to through the ball far only if he pushes against the ground with his legs. Then, the energy flows thought the body until the finger tips where he releases the ball.
In order to feel the same in tennis, it’s vital to play with your base a little bit lower than usual, and that you keep your bottom soft.
The timing of your shot will mainly depends on the moment that you have your stance before the shot. If you find your stance to late, the energy won’t able to through your body for the moment of the shot.
To sum up
You don’t need to have strength, to have a powerful tennis shot. It’s the beauty of this sport. Any one can play. More over, with the modern tennis racket (lighter and manipulable) tennis becomes more accessible.
Rather you need to improve you reaction time, in order to have time to move and to prepare yourself for the shot.
Coordinate movement and hand skills are far more important than strength in tennis. Strength is needed in your core muscles and back, to hold the balance during, in order to have accuracy but it is not required to generate a powerful shot.
Hope it helps.