Helps keep idle steady. Will most likely drop it. Not to often if your idling funny, really only time I'd do it.

 

so its like a breather or something cuz when i start my car it goes up to like 1700 then drops to 800 or so then when i start driving or when its warm its like 500 or so is that where it should be

 

Yea, its good. Or at least thats what mine does too. I idle at like 700-800 all the time.

sleeping stallion, I just asked about it too in an earlier thread and someone told me about it. Its a hose that bypasses the throttle body so the throttle doesn't have to be open to idle (basically so you don't have to hold the pedal down to idle), it feeds the engine air at idle. I didn't even think of that until someone told me. I just noticed it too on my intake, a piece branches off of my intake and the hose goes right around the throttle body to the plenum. Check it out.

 

You will find out that cleaning the IAC is kind of a one time deal. My idle was really rough so I fully cleaned the IAC, a week later the idle got rough again and the rev's would drop so low that my car would stall. When it gets that bad you will need to shell out the $50 and just get a new IAC.