There are reasons why the black end cap would come flying off the spindle:

1.  Check if the black end cap is properly installed.

2.  If the black end cap is installed, 2 things to consider:

      – the black cap you have could be loose. We always enclose a small paper envelope of spare parts containing the retainers and elastic bungee.

Please use a new one, or preferably that you use the spare PVC tube instead of the black end cap.

– Your racket tip is too close to the TopspinPro – see this pic for an explanation.

When you strike the ball and cause lateral force, it will easily come off.  In cases like this, we are suggesting that customers use the PVC tube instead.  There are spare PVC tubes inside a little ziplock bag that you may use to replace the black end cap.  Please let us know if you need more.

Here’s the assembly guide to help you.