That screw does it all when it comes to setting idle speed.

when you adjust the idle screw on an oem tb you change how much air the tb needs to bypass/add to maintain idle speed. this aftermarket tb doesn't have that idle adjusting screw. There is an air passage that will allow air to get into the plenum when the butterfly is closed, but in my case it wasn't enough air to get a 900 rpm idle. So to increase idle speed i had to turn the screw cw which opened the butterfly and gave me more air.