The output side is not necessarily directly linked to the input side.
This is probably what is happening:
A digital signal is first fed to a F-V converter. The output V from this is being fed to a comparator circuit. The comparator circuit tells the "chopper" mosfet driver the speed and direction to drive the motor. The chopper drive is independandly operating at 20 Khz.
A basic description of how this is done can be found via the application notes for a couple of IC's - LM2917 F-V converter and UC3637 motor controller. Although the 3637 is a 10khz chip, the operation is essentially the same for a 20khz chip.
Anyway, feed that 100Khz max input freq into the 2917 (or one suitable to operate at that freq) and out comes an analog control voltage that is proportional to the input freq.
Feed that voltage to the input comparator of the 3637 and you ultimately control speed and direction of the motor.
Pretty simple, huh???