When the primary throttle bores are opened fully, the secondary throttle plates and bores are also activated, allowing double the air/fuel flow over the primaries alone. The secondaries open mechanically as the primaries reach wide-open-throttle. To prevent hesitation, Edelbrock has installed an auxiliary air valve above the secondary throttle plates. With the 1400 Series carburetors, this air valve is weighted to stay closed normally. When we open the secondary throttle plates, the inrush of air counteracts the weighting, opening slowly to prevent hesitation. The Thunder Series AVS carburetor uses an adjustable spring with this air valve, which allows you to adjust the rate that it opens.
When the secondary throttle plates are open fully, fuel is drawn to the boost venturi the same way it is on the primary side. Fuel flows through a metering jet at the bottom of the fuel bowl, rising up the secondary well-tube to the boost venturis. the primary metering circuit has metering rods and jets; the secondary metering circuit has jets only. Instead of an accelerator pump on the secondary side, fuel is drawn to the booster in liquid form through a brass nozzle, which controls hesitation.
The cold starting system is the choke--that butterfly at the top of the carburetor. As its name implies, the choke cuts off the air supply through the carburetor, allowing more fuel to flow through the idle and main metering circuits when the engine is cold. As the engine warms, a thermostatic, bimetallic coil in the choke assembly (automatic chokes only) expands, pulling the choke off. This coil is heated with either an electric heating element or exhaust manifold heat drawn up from the manifold or header below. Intake manifold vacuum is what draws the heat from the manifold or header to the coil.
You can swap the jet sizes, as well as the metering rod sizes, based on expected demand. Just follow the Edelbrock table in the owner's service manual. | 
The fuel metering rods control fuel flow through each of the primary jets depending on demand. | 
The metering rods do the same job as Holley's main metering jets and power valve. |
Fuel is drawn into the idle circuit via the idle air bleed and metered by way of the idle jet. The transfer slot is what makes the transition from idle to power circuit smoother. It also provides fuel during part-throttle conditions. The idle screw controls fuel flow through the idle circuit. | 
As you can see, the Edelbrock 1400 and AVS series carburetors meter fuel through the main metering circuit via metering rods and jets. The main well bleed and tube help channel the vacuum to the primary well-tube, which draws fuel to the booster (or boost venturi). The step-up piston attached to the main metering rod rides on intake manifold vacuum and a spring. When there is low vacuum, the spring pressure overrides vacuum, lifting the metering rod out of the jet, which allows more fuel to flow. When manifold vacuum is high, vacuum overrides spring pressure, drawing the step-up piston down, sending the larger end of the metering rod into the jet and allowing less fuel to flow through the jet. | 
This is the secondary main metering circuit at wide-open-throttle. |