 Fuel is introduced into the air stream through small holes at the narrowest part of the venturi and at other places where pressure will be lowered when not running at full throttle.Fuel flow is adjusted by means of precisely calibrated orifices, referred to as "jets", in the fuel path.As the throttle valve is opened slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate/Valve blocking the air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metered fuel flow through the regular open throttle circuit.