Fluid speed increases when the fluid is forced through a narrow or restricted area. The increased speed results in a reduction in pressure. The underbody venturi is shaped to create a low pressure area between the road and chassis which creates downforce.
as a fluid (such as air) flows through a constriction (like a mountain pass), the speed increases and the pressure drops
Increase in the speed of flow of gasses or liquids caused by a constriction or bend in the containing duct.