How does an aspirator pump use flowing water to draw in air?

An aspirator pump functions by utilizing the principles of fluid dynamics to create a vacuum. When water flows through a venturi nozzle, the velocity of the fluid increases, which results in a corresponding drop in pressure.1

This localized area of low pressure creates a suction effect that draws in air from an inlet line, entraining it into the water stream. This mechanism is a specific application of the jet ejector principle, where the kinetic energy of a primary fluid is used to induce the flow of a secondary fluid.1