By progression we mean the transition period between mixture delivery from the idle circuit and the beginning of mixture delivery from the main jet circuit.
On first opening the throttle, the air drawn into the engine increases and therefore, in order to have an inflammable mixture still, the fuel supply must also be increased.
As previously noted, the idle hole(3) shown in figure 20, only delivers sufficient fuel for engine idle operation and the main circuit still does not deliver any fuel because of insufficient vacuum up stream of the throttle. The progression hole (2) is therefore necessary to deliver the fuel required during this transition period. The progression hole draws fuel from the idle circuit (4) and is positioned immediately upstream of the closing edge of the throttle slide (1) for the promptest response to fuel demand when the airflow suddenly increases.
It is interesting to note that the progression hole serves a dual purpose: When the engine is idling, air from the main barrel passes into the progression hole and weakens the mixture flowing through the idle circuit; When the throttle is opened slightly, the idle circuit mixture flows into the main barrel through the progression hole.
The progression hole therefore first feeds air in one direction and then feeds mixture in the opposite direction.