https://www.bostonglobe.com/2021/05/27/sports/how-red-sox-reliever-garrett-whitlock-is-making-necessary-adjustments-hitters/
Very good article on Whitlock...working on his slider...
For Whitlock, while his changeup-sinker-four-seamer mix is impressive, in order for him to take that next leap they know he will need another pitch. Despite working at different speeds, his changeup and sinker have the same arm-side run. As a result, big league hitters can sit on a location but still adjust to velocity.
So, Whitlock has been working on a slider, just to give hitters a different look on a pitch that, ideally, runs in an opposite direction. In addition to direction, Whitlock is trying to adjust the speed of his slider.
This is type of information I wish more writers would write about....