I think it produces a controlled-carrier effect, which can be very annoying at the receiving end, with certain receiver agc characteristics.
No, the carrier doesn't change., the bias to the Modulator tube changes. The average plate voltage on the RF stages says the same as with standard heising modulation. At low audio levels the current thru the modulator tube is low, but as audio level increases the bias to the modulator is reduced to increase the current thru the modulator tube so the voltage across the tube can swing further.
The bias voltage is changed at a sylabalic(?) rate, the idea being that most of the time the modulator tube current can be low, thus reducing dissapation.
The only thing a listener might hear is that as the bias on the mod tube changes, there will be some low frequency 'thump' as the bias goes up and down. However this will probably be well below the audio response of most receivers.
Ian VK3KRI