By replacing the existing reality with my personal reality. All I have to do is touch a vector—any vector—and I can freely change the magnitude and direction to whatever the hell values I want it to have.

[That's about the briefest rundown on how it works that he can give without getting into detail on quantum mechanics, AIM diffusion fields, the massive equations espers need to do in their head in order to manipulate the particular aspect of reality for which they have an affinity, etc.]