Unless I'm missing something I think there is a display error.
From the Select Skill Test page when Topspin vs Block is selected, and then Start Exercise is clicked, the Topspin vs Block page is displayed, but the instructions under the table image reads:
"Serve with as much backspin as possible. The ball needs to bounce two times on the partner's side to count."
This incorrect instruction is displayed for all of the skill tests (of course it's the correct instruction for the Short Backspin Serve Exercise).
Obviously a minor error, especially since the correct instructions are displayed on the page displaying the Start Exercise button.