It is so simple you can't believe it. It's only Ohm's Law.

The Class-C plate modulated final amplifier is driven hard on its
control grid such that it is driven into saturation even when its DC
plate volts is increased to twice its normal DC value.

When saturated, the peak RF plate output volts always swing between
twice the DC supply volts and some very low value.

The final amplifier therefore constitutes a fixed DC load resistance
across the DC supply, which depends on unmodulated RF power output and
amplifier power efficiency.

This fixed resistance is also the value of the audio-frequency load
impedance across the secondary of the modulation transformer.

For 100 percent modulation, the peak RF plate volts across the tank
circuit swing between twice the modulated plate supply DC volts and
zero. (Or some very low value.)

The harder the grid drives the plate circuit into saturation, the
greater the modulating linearity. The drive limit is reached when any
of the tube's electrodes approaches its rated power dissipation.

Eaxactly the same things happen with transistors.
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Reg.