Transition or Progression Circuit
When the operator starts to open the accelerator, the throttle valve opens and therefore decreases the vacuum that in the closed condition activated the idle circuit. The delivery of fuel from the idle circuit is reduced, and therefore it is necessary to introduce another system, which is able to handle the transition of functions from the idle circuit to the main circuit. This circuit is called the progression system. In the process of opening the throttle, the vacuum is sufficient to draw fuel from the progression ports, which are positioned upstream the throttle valve when in fully closed position. When the throttle valve is open slightly the vacuum generated by the inducted airflow begins to be consistent, and stops drawing fuel from the idle nozzle. The progression ports are always fed by the idle jet placed on the side of the carburetor body. This explains the importance of the idle jet, even in the first stages of throttle opening. The position of the progression port, between the main and idle nozzles, is very important for the correct operation of the carburetor and is the subject of careful development. Because the engine has to quickly accelerate, the progression system mixture of air and fuel is very rich.
See movie below: