Throwing inside? Check the feet
One of the common problems that crops up for pitchers is a tendency to throw inside, i.e. right handed pitcher throwing inside to a right handed hitter. While there can be any number of causes, one I’ve seen a lot is the throwing arm side getting in the way of the throwing arm. When that happens, the pitcher tends to push the ball away from that side in order to avoid hitting her hip and the ball goes inside.
If you’re seeing that, one thing to check immediately is where the pivot foot is going as she drives forward. (In case you don’t feel like thinking, the pivot foot is the foot that is on the throwing side. You’re welcome.) What you’ll probably see is that the toes of the pivot foot are going toward the toes of the stride foot after the latter lands. Often you’ll also see a walk-through, i.e. the pivot foot will keep going past the stride foot.
The simple correction is to tell the pitcher to take the toes of her pivot foot behind the heel of the stride foot. When that happens the hips stay out of the way, the arm stays on the power line, and the ball goes where it’s supposed to — usually. If nothing else it will go a lot less inside and will improve over time.
The nice thing about this instruction is it’s simple and specific. It’s not that difficult to take one foot behind the other, yet it can have a significant effect. Then all you have to do is remember to watch that the pitcher keeps doing it.