The position of the servomotor and hence the amount of air bypass is controlled digitally by the engine ECU.
The linear servo is most commonly a combination of a DC motor, lead screw and a digital optical encoder.
The IAC allows the ECU to maintain minimum RPM irrespective of changes in engine load, sometimes referred to as anti-stall feature.
The most common failure mode is partial/complete jamming of the actuator (due to dirt/dust or even oil) where it cannot be smoothly controlled.
The result is an engine that fails to maintain idle RPM and frequently stalls.