When a carrier is amplitude modulated, the output power of the transmitter
varies. What happens if the output power varies to the point that the power goes
to zero? The transmitter stopps sending any signal whatsoever for the moments
that the modulation cause the power to go to zero. This is overmodulation. This
is like clipping distortion (see amplifiers in Analog Circuits) and produces
undesired harmonics. Not only does it sound bad at the receiver but produces
side frequencies that are out of the channel assigned by law.