What happens when idle air control valve on a car goes bad?

when your vehicle is in park the rpms will rev and and down fluctuating. howver, when car is in gear it will seem to be fine. but, as problem gets worse it will shift to higher gears. for ex. when mine went out my car maintained 45 mph without doing a thing. also, when you go to slow down it will jerk because it is having trouble switching to lower gears due to air intake.