What is the key sentence?

"Negativism isn't a philosophy, it's an attitude. It's the attitude of a player whose nerves aren't as strong as he'd like them to be. Attitudes can be changed, but first they have to be recognized. It's quite common for a player to think to himself, "I don't want to boot this ground ball," or "I don't want to walk this batter." The word "don't" will not get through to the body. The word carries no functional image. The phrase "boot this ground ball" does bring forth an image. The expression of a negative goal will therefore emphasize an undesirable image and the error or the walk is more apt to be made. The body tends to do what it hears most clearly; the mind tells the body what it sees most clearly. So, thinking about what you don't want to happen greatly increases the chance that it will happen."

The above is from the book I read. I wonder not only what is the key sentence bit also the topic.

Thank you very much in advance.