The goal of tennis ball massage is to achieve a “release” by applying just the right amount of pressure: enough to do some good, but not enough to irritate your nervous system. (Although it’s quite safe, massage is not harmless.3) The sensation should be clear and strong and satisfying, what we call “good pain.”
If you are wincing or gritting your teeth, please be more gentle with yourself. You need to be able to relax.
Once you have adjusted yourself to achieve the right pressure, relax as much as possible and wait for the sensation to fade to about eighty percent of the original intensity. This is the “release”: a change in the physiological state of the tissues, or a “melting” of the knot, and/or just a sensory change, relief, a subjective experience of it easing. This can take anywhere from ten seconds to a few minutes. I’ll briefly introduce the (messy) biology of this below.