The curvature equation of Seiberg-Witten equations is $F^+_A=\psi\otimes\psi^*-\frac{|\psi|^2}{2}Id$, where $F^+_A$ is selfdual part of curvature and $\psi$ is spinor.
Could someone elaborate why could we subtract the terms and get a bound $|F^+_A(x)|=|\psi(x)|^2/2$?