When I work with pitchers in a practice setting or on the side, there are generally two things I try to never let happen:
Pitchers generally reach fatigue when they have thrown 25-30 pitches in a row.
You can usually see it in their mechanics and with their command of the strike zone.
Since I love to see pitchers get out of an inning in around 12-15 pitches, I usually have my guys work in sets of 15 in the bullpen, with breaks of 3-5 minutes in between.
This allows them to get a feel for an economical inning and also keeps the bullpen fresh.
If a pitch such as the slider or change up isn’t working on a given day for one of my guys and we have tried a few different things to fix the issue to no avail, I’ll usually take the pitcher off the mound and we’ll do some work in the outfield instead.
This accomplishes a three things:
This tends to divert the pitcher’s attention from a troublesome pitch and will often times fix the problem.
Then we can go back to the mound with confidence.