From the LTA coach education resource...
Ball Characteristic: Height
A more level path will keep the ball on the same height. A low to high path will send the ball higher. A high to low path will send the ball lower.
Opening the racket face angle will send the ball higher. Closing it more will send the ball lower.
Increasing the racket speed can also increase the height of a shot if the racket face and path are angled 45 degrees or above. Decreasing the speed lowers the height of the ball.
The amount of topspin can be increased or decreased by changing:
• The racket path
(a more upward path will increase the spin)
• The racket speed at impact
(more racket speed, will increase the amount of topspin)
(Language is simplified (as tennis coaches are a bit simple
) but is based on research evidence from Cambridge University Cavendish Laboratory and the Institute of Physics)