Stock ECU has ignition ramp to stabilise idle, thus when TPS is at 0.45v aka throttle off.. closed loop idle/o2 is enabled - this is why on a stock ECU when you're trying to set your base timing you'll notice it jumps all over the shop.
PowerFC I believe does not have ignition ramp to stabilise idle, so there's no need to unplug the TPS to set base idle and/or the base timing.