Even though this article is about baseball pitching I think most of it correlates well with fastpitch and does a good job of describing the role of the arm in a pitch.
https://www.topvelocity.net/study-proves-arm-path-most-effective-for-increasing-pitching-speed/
https://www.topvelocity.net/study-proves-arm-path-most-effective-for-increasing-pitching-speed/
