Originally Posted by claes
This is exactly what I don't understand - thank you all for the discussion
Circuitry on the pwm device interprets the control signal and uses that to regulate the power driven to the fan's motor. There are lot's of variations on how to go about doing that, lots of details. How to interpret 0 duty cycle for example, it's never interpreted as completely "off" for fan pwm, but what level of not off does it mean? There's also how the power is driven to the motor, power delivering current is ultimately pulsed, but the shape of the pulse wave forms can be tweaked.
This coolermaster advertisement has as a simple diagram showing different pulse wave forms and bragging about how their driver makes things quiet.
The hardness or softness of the pulse edges can indeed have a significant impact of motor noise, the clicking sounds. The vid below isn't from fan control but demonstrates the sort of impact that tinkering with the shape of pulse edges can have on stepper motors
How pc fan's work tutorial, pre-requisite to understanding how the driver ic fits into things.
The spec sheet for a pwm controller driver ic, the sort of device that might be found on the pcb board of a pwm fan. See figure 13 for a diagram of what it does.