After reading about the serve and practicing it for decades I learned about internal shoulder rotation in 2011 in a TW thread and some earlier threads by Toly. It was a shock to me.
The biomechanical terms are good because these take each joint and name the motions that the joint is capable of. Therefore, internal shoulder rotation is an axial rotation of the upper arm. You can search it on the internet.
What the body is doing to get high racket head speed is taking the largest muscle attached to the arm, the lat, along with some others and rotating the smallest possible mass to get a very high racket head speed. The smallest mass is the straight arm axially rotated plus the light tennis racket held at changing angle as the arm rotates. This is shown very clearly in all the Vimeo pro serves.
You can also compare my puny internal shoulder rotation and see why I have the serve that I do <90 MPH and the pros have theirs >130 MPH.
Average players naturally do ISR to some extent. https://vimeo.com/55660219