Pulse Width Modulation (PWM) is a technique of creating high speed digital pulses that have a variable amount of ON time vs OFF time within each pulse. This results in the ability to control a final load in a fashion that is similar to analog control resolution with digital control outputs. The Power-io HDD family of products is capable of achieving PWM speeds up to 15 kHz.
The following article by Michael
Barr provides an overview of the technology.