I've been telling my hitting and pitching students for years that negative thinking - thinking about what you don't want to happen - has no place when you are trying to succeed at anything. If a pitcher is going into his windup thinking "Don't throw it high," what's he going to do? Throw it high, because that's what his brain was focused on.
The only way to get that negative thought out of his head is to replace it with a positive thought - like what he does want to happen - "throw it low." He can also think about the mechanics involved in throwing a lower pitch: get the chest out, reach out for the target and follow through low. Now his brain is focused on the result he wants and he has a much better chance of getting it.
The mental side of baseball plays a huge role at the youth level, especially on the mound. A player can have the best mechanics in the world, but unless he knows how to think positively, he may not get the results he wants.