If the body doesn't know "not," then what happens when we say a sentence like, "I'm not a good golfer?" Does the body hear, "I'm a good golfer?"
It does not. When we say that the body doesn't know not, we are saying that an affirmative statement is met as concretely real in the body, whereas not is an abstraction, which takes it out of the realm of the body and into the realm of the mind.
We cannot make energy flow by using the mind. Energy flows by moving to center, by connecting to the open, flowing breath, and allowing the breath to draw energy into and through us. We cannot think our way to flow.
However, we can definitely think our way out of it. When we say a sentence like "I'm not a good golfer," we're using the power of the mind to clamp down on flow. The essential guiding hypothesis of TTW is that athletic ability (any ability, really) depends on an open, unfettered flow of energy. Any blockage in that flow will inevitably limit one's abilities.
Thus, no matter what our skill at the sport, saying, "I'm not a good golfer" becomes true to some extent just by saying it.
Don't take my word for it. Try it for yourself. Center. Establish an open, flowing breath until you can feel the energy flowing. Now, pick something that you are working at meeting your potential in. It can be anything, just as long as it's something you care about. Now, say to yourself, "I'm not a good _______." What did you notice about the breath? What did you notice about the energy in the body? Did you find that the breath didn't flow as cleanly (if at all), and the energy in the body closed up or fell away?
For now, try to start being aware of times you use language to limit your flow. Awareness is the first step to changing that pattern.