The overall idea is to aim small and miss small. So if you have a pitcher who needs to gain more precision in hitting her spots, give this drill a try.
A few months ago I put up a post that showed a way to help fastpitch softball pitchers who were struggling with hitting their inside and outside spots by exaggerating the locations.
The idea is that by making the adjustments larger you can help them get a feel for what it takes to move the ball from side-to-side.
Here’s another way to do it, using kind of the polar opposite approach. This is more for fine-tuning, when the pitcher is already pretty good at going inside/outside but you want to make it more precise and reliable. All it takes is some scrap wood and a couple of pool noodles.