Changing the IFRs and IABs changes the air/fuel ratio of the fuel entering the idle circuit, why it is recommended to only change one so much before the other needs to be changed. By keeping these constant and at a good ratio you can restrict the slot itself which measures at about .082 in area to get the desired result, plus you can use a smaller restriction on the primary side for lean cruise afr and something a little larger on the secondary where it is not needed.
Here is the full story for those time inclined:
http://speedtalk.com/forum/viewtopic.php?f=1&t=36018