I had a major neck surgery 10 years ago, during which my spinal accessory nerve was damaged, creating a whole host of muscle issues in my upper back/shoulders. I did some physical therapy after the surgery, but still experience a lot of muscle tightness and "knots" ever since. A lot of them seem to arise in my trapezius muscle, at the bottom of/slightly under my left scapula. The scientific literature on the topic of "trigger points" and myofascial release does not seem to be concrete or well-understood, but I am curious about this kind of muscle therapy and what others' experiences are with it. I do occasionally use a small tennis ball to apply pressure/massage my upper back and shoulders, and I will sometimes land on a spot (or a "trigger point") where I experience a "good" sort of pain. Oftentimes when I do this, I can feel the muscle start to twitch like crazy, and it will continue to do so for up to a minute or so sometimes. It really does seem to make the tension in the area vanish. From what I've read, it seems this is a nerve-related response. I have read about dry needling, where they stab needles into the muscle to elicit this kind of response, but that seems a bit more extreme to me.
This guy appears to have created a tool to apply pressure on his muscles/trigger points, and it gives a visual of the same kind of twitch response that I've experienced.