It's a potentiometer that supplies the with ECU info on engine load, I.E at WOT the ECU at other end when your off throttle it can shut off fuel on the overrun for better economy. knows from this it needs to provide more fuel. Nothing related to being Turbo N/A cars have them. Presume RST uses a ISCV for idle compensation if so if this is set wrong it will give false information which is why your idles all over the place.

Martin