Just like in the cases with Top Spin Swings, I believe there are “Clean Responses” for Under Spin Strokes to be developed and conditioned in players. But before explaining these Under Spin Responses, I think it is important to understand why they need to be different and have some grounds for it.
The difference starts in the nature of the impact made on the ball by the racquet. Differently from “snappy” Top Spin impact, Under Spin impact is typically made by “punching” or, the way I like to call it, “notching” motion, especially when volleying. At least, that’s how I recommend to do it. Even during swinging through the ball on slice ground strokes, racquet works better with wrist remaining “cocked” all the time. It’s the fist motion going through the ball that adds length and extension to the swing path. I understand there may be exceptions and other options depending on situations in which slice is used , I am just pointing at the main difference between spins productions.
Another concept that helps to understand the main difference between the two spins is that the time needed for Top Spin is greater than time needed for Under Spin. Therefore, the Under Spin Motion is more compact and quicker than Top Spin motion, as a general rule, and particularly for volleying.
So “Clean Under Spin Response” needs to stay as much as possible in front of the torso, fist wise and racquet handle wise, yet, not as far as possible with arms extended straight forward. Even when the racquet head is slightly behind the shoulder, it is better when fist and racquet handle stay in front of the body. The racquet face gets open to a different degree angle depending on the height of the contact point with the ball.
Continental grip is huge for effective and “Clean Under Spin Response” and used by “holding the racquet by the spine,” as one would when “holding the hard cover book by the spine.” It is absolutely neutral and allows even access to openness of the racquet face on both sides without favoring one side at a time, thus eliminating grip changing from forehand to backhand when time is very limited, as was mentioned earlier.
These are some crucial differences that allow player to use the same tool of the tennis racquet to generate different spins on the ball. Of course, there are more differences in Top Spin and Under Spin productions, and more details about these and other ones, but I want to nut shell them as much as possible to have some grounds for “Clean Under Spin Responses.”