When the throttle valve is opened past the idle and low speed ports, it allows air to start flowing through the air horn. This will build a vacuum in the venturi, and the main discharge nozzle will begin to feed fuel.
At this time, the idle and low speed ports are still feeding fuel.
As the throttle valve opens further, venturi vacuum becomes quite strong, and the main nozzle supplies ample fuel. With the throttle in this position, vacuum is lowered at the idle screw port, and both it and the idle port will stop supplying fuel until the throttle is closed again.
Notice that the throttle is well open, and there is a strong vacuum in the venturi. See how the fuel is being drawn from the bowl through the main discharge tube metering rod jet (which controls fuel flow). Fuel is then pulled up the discharge tube and out the nozzle where it is thoroughly mixed with the passing air.
