High-power ferromagnetic resonance in a flat plate has a small component of the precession parallel to the applied dc field. This component causes frequency doubling and also coherent amplification of some scattered spin waves. These spin waves then cause a linear damping of the parallel component of the precession, which in turn causes a nonlinear damping of the principal component of the precession. The nonlinear damping causes the microwave susceptibility to decrease with increasing power and to become approximately inversely proportional to the rf field at high power levels.