They are set at around 75 degrees which is the optimum angle for learning the topspin component of the swing. Groundstrokes actually consist of two components:

1.The spin Component: either up (topspin) or down (slice) swing = controls spin level and depth

2. The forward swing = controls power and also depth

Each groundstroke you hit will be a unique blend of the two depending on what you want to do to the ball. The TopspinPro purely teaches the up swing or topspin component and allows people to develop muscle memory. If you would like to add in the forward component too you can remove the top screen and that allows you to practice a variety of swing shapes.