ABSTRACT
The key to the difference is the phase angle. When we multiply two sinewaves in phase with each other we get a third sinewave of twice the frequency, and it lies entirely in the positive quadrant of the X-Y plane. But when we multiply two sinewaves that are 90° out of phase with each we get a third sinewave of twice the frequency, half of which is in the positive quadrant and half of which is in the negative quadrant. Its average over the entire cycle is zero.
