3. Speed, Frequency, Topspin and Slice balls
All trainings, which could be set on the ball machine, can be changed for the speed. Also, just like a
tennis player, the ball machine can hit the balls with less/more speed or with
topspin/underspin(slice). The spin has a fixed speed, less topspin let the ball hit the ground more
deep and more topsin let the ball hit the ground more close to the net. The speed and
topspin/underspin couldn't be mixed together. When changing the speed, during topspin/underspin,
the topspin/underspin will be stopped.
