The Tennis Tutor Plus Tennis Ball Machine includes all the features of the standard Tennis Tutor, plus adds topspin and underspin capability. A simple dial controls the amount of topspin or underspin you desire. The Tennis Tutor Plus measures eight inches taller than the standard Tennis Tutor.Tennis Tutor Plus is the only portable ball machine that displays the Elevation setting right on the control panel. A simple 10-segment display shows the elevation setting so you can set it to your favorite spot before firing any balls.