How is the IAC valve turned on/off?

I am trying to diagnose an issue where the car has unsteady idling. I noticed when I unplug the IAC valve the idle it fine, so I assume the IAC valve is no opening at the right time. I know the IAC valve works as I can feel in actuating - it just doesnt' seem to work at the right time, or to stay on.

So, I want to inspect what makes the IAC turn on and off. I traced the wiring back to behind the fire wall but lost it there.

Would anyone know what makes the IAC turn on and off?