It's been a long time since I looked at or worked on a Q-jet, so I'm going from memory here. If I'm wrong, I'm sure someone will jump in and correct me...please do!
As I recall, the secondary throttle blades on a Q-jet are opened mechanically, and they can only be opened when the choke is fully opened. There is a metal plate that covers the secondary venturis, and these are somehow vacuum operated(I'm sure it's way more complicated than that), and open progressively as the engine requires more fuel/air. This plate is connected to the secondary metering rods..as the plate opens, the rods are pulled upward and more fuel is added.
Like most carburetors, it operates on the same basic principle as your toilet, with a few extras thrown in.
I think the vac cannister you see on the primary side is a choke pull-off...it keeps the choke valve from completely shutting off air to the primary side. Also called a vacuum brake.
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