You aren't really taking in any more air than the standard manifold, what you're doing is altering the flow characteristics, (or "harmonics" as already mentioned). The same as with a 4-1 exhaust manifold for example, you aren't getting rid of any extra exhaust gas, you're just freeing the restriction that comes with the standard manifold, which gives you power gains in certain areas of the rev range (usually up top for a good 4-1).