The middle of a Venturi tube is narrowed, causing acceleration. This is essentially what necking down a certain cartridge does, the reason that (everything else being equal, which admittedly it rarely is) smaller-bored rounds on a given case tend to create higher muzzle pressure.
The far end of a Venturi tube is of larger diameter. The differential in velocity between the two diameters creates a pressure differential that's a measure of flow. The "venturi effect" is commonly used when talking about accelerated flow of gas or fluid through a smaller tube.