It is for the power steering. Basically it raises the idle to help turn the tires while the truck is not moving. At least thats what I've been told.
Yup it is connected to the p/s pump to raise the idle when you are turning so the engine doesn't bog down under the increased load of actually turning the tires while parked.
Basically it creates an adjustable and controlled vacuum leak. You turn the knob to adjust how big the leak is...
