Yay, a question where studying several years of fluid mechanics gets to pay off?!? I knew I wasn't wasting my life...
What happens is that there is a constriction in the tubing, and when a fluid (gas or liquid) has a higher velocity, it has a lower dynamic pressure. Not a vacuum, but the lower pressure does pull air in. It is called the venturi effect since it is based on a venturi meter, which in a pipe a constriciton is intentially placed, and the pressure drop from the restricted to the unrestricted pipe can be used to measure how much flow is going through that pipe.
Has anyone every used an aspirator in chem lab? Thats one of those thing you attach a forked tube to the sink, and when you turn the sink water on, the water, making the pressure lower in the pipes, you get a large amount of suction on the fork that doesnt have water in it? A cheap way to work under low pressure and to dry precipates from a liquid based reaction.
And, all of this is decriped by Bernoulli's equaiton a.k.a. Bernoulli's principle.
And, in extra bonus converage, Bernoulli's principle is not the dominant reason airplanes fly,Sean, it is the flow turning. The air that hits the wing straight on is turned slightly downward, and that is the major reason airplanes fly. The pressure difference predicted by Bernoulli's principle helps, but it not the dominant effect.
In super extra bonus coverage, those of you who are of age, next time you pour yourselves a Guinness beer, notice how the bubbles right next to the glass appear to move downward. This is because all the bubbles in the middle move upward, pulling the liquid beer up with them. When the liquid gets to the top, it has to move out of the way for the next slug of liquid moving up so, liquid moves back down the side of the glass. The bubbles on the outside of the glass get pulled downward with this liquid, that is why the bubble appear to move downward.
This is how the undergravel filters work: the airstone bubbles pull the water up, which means that the water has to go down through the gravel draging the wastes into the grave with it.
Actually, if I've got it right, it's where you stick a little bit of airline onto a connector on your filter outlet, the top of the airline goes up into air, and it basically mixes air into your filter outflow, causing fun bubbles and aeration. Most of them have a little twisty thing on the top, letting you adjust how much air you add.
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