Just recently read a study regarding the relationship between pitch speed and exit velocity. The old the faster it comes in the faster it goes out thing. Turns out there is only a very small relationship between pitch velocity and exit velocity. I should say direct relationship more accurately. The reason it seems like this is because most hitters especially at the amateur level tend to slow their swings on the slower pitches and turn and burn on the faster pitches. Its a natural reaction for many. Sometimes hard to get kids to wait that tick or two longer and then swing top speed.