Most tennis ball machines on the market utilize the counter-rotating wheels method. Inside the tennis ball machine are two wheels that turn in opposite direction. The wheels have a small opening between them where they affect the incoming tennis ball.
The tennis ball then rolls down from the holding area. It travels from a tube to the space between the rotating wheels. It is squeezed under tremendous speed and pressure.
The ball is then ejected via the immense speed and pressure from the counter-rotating wheels surrounding it. Tennis ball machines which use the wheels can alter the spin by changing the spin of the wheels. If the upper wheel is spinning faster, the ball ejects with topspin. Backspin is accomplished by spinning the lower rotating wheel faster than the top.