Just as a disclaimer... I highly recommend for other coaches to NOT use this exercise. This is some risk involved. If you hit their eyes, you could be in huge trouble.
Many of my students don't bend their knees. Many of them bend their knees at the start of the point, then begin to stand up or bounce too much during the long rallies. I have been threatening them for years that I would add an adjustable drop-ceiling to my tt room. Ok, I finally did it. Now, if they stand up, they are reminded by the drop ceiling.