[jonnyk]

The idle problem it is intended to fix is too little air flow at idle on modified cars. This causes a surging idle which can get quite extreme to the point where the car will stall (mine used to). The IAC valve doesn't offer enough airflow for bigger engines and/or highly modified engines. The bypass valve adds more airflow that is user selectable via adjustment screws - you can set it to get the car to idle, and you won't have to crack the throttle blade to do it.
It can be had at a Ford dealer part # F2PZ-9F939-A
I think I paid around $25.