The opposite of phase cancellation. An reinforcement of signal components resulting from combining phase coherent waveforms. When two waveforms are mixed, their harmonics are added. If these signals are not perfectly in- phase with each other, the amplitudes of the harmonic components differ at various times, (as determined by the phase relationship.) If the added harmonics have the same polarity, the signal is reinforced at those frequencies.
Phase Response the first derivative of the amplitude.