It all depends upon which carburetor you're talking about, but if the carburetor rotor moves in and out when you throttle up and down, then what happens is that the idle adjustment changes the area of the fuel outlet inside the carb venturi. Some carbs have an actual idle needle, and the taper of the needle does the job. Other carbs have a slot that is opened or closed by a sliding tube when the throttle is opened or closed. Some of the high-end carburetors have a specially-shaped slot that exposes more or less area as the throttle opens and closed.

In all cases, the adjustment of the idle mixture adjusts how much fuel is permitted to flow at idle. The shape of the needle or the shape of the slot determines how the mixture changes as the throttle is opened or closed.\