It is very interesting to observe how two very different strokes produce ball velocity in tennis.
One is the groundstroke, a very different one is the volley.
How do you speed up a groundstroke? The most efficient way is by accelerating the racquet.
How do you get ball speed on a volley? By stopping the racquet.
Why is that effective? Well, remember Newton’s second law? Force is mass times acceleration.
On the groundstroke, you apply more force by accelerating
On the volley, you apply more force by increasing the mass. In this case, to stop the racquet you tighten up the arm and pectorals, thereby increasing the mass (weight) behind the stroke.
From the weight of your racquet on the groundstroke (assuming you play like a pro, with a loose grip), you increase the weight to include your body’s own, assuming you tightened up your grip with a stopping action which thereby will connect the rest of your body to contribute to this stop.