According to About.com Scuba Diving, the Venturi effect states that when air is forced through a narrow tube, the air particles leave the tube traveling at a greater speed than when they entered the tube. This results in the formation of a vacuum at the end of the tube because the fast-moving air particles tend to drag some slower-moving air particles along with them.
The Venturi effect also occurs in liquids. Liquid that is forced through a narrow tube increases in speed along the length of the tube, leading to a reduction in fluid pressure at the end of the tube. This effect is named after Giovanni Battista Venturi, the Italian physicist who first described the phenomenon in the 1700s.
Many industries take advantage of the Venturi effect when designing equipment and machines. Automated pool cleaners, for example, utilize the Venturi effect to draw debris and sediment into collection tubes. Sand blasters also utilize the low-pressure area created by flowing air to draw sand into their systems and mix it with air. Scuba divers in particular rely on this phenomenon. The latest breathing regulators are constructed with flexible diaphragms that are held in place by the vacuum created when the diver breathes in, forcing air down a narrow tube.