Most importantly, it removes the material that would otherwise become fuel for excessive algae growth. Secondly, it cleanses the water which raises the redox potential. It also acts as a mechanical filter, removing particulate matter and detritus. The airstone is placed at the bottom of the skimmer, with the hose extending up through the top of the skimmer. The fine air bubbles will cause the water to rise through the skimmer, while the organics are precipitated into the collection cup.
Air driven skimmers are great for smaller tanks.
The airstone sits at the bottom of the skimmer, just as it does with the air driven units. The difference is that a water pump feeds water to the skimmer from the top, and is pushed downward toward the rising air bubbles. It is this opposing action that causes a bit more turbulence, making it a bit more effective. A true venturi skimmer pushes water through a very small hole at high velocity. At the same time, air is drawn into the opening which is sucked in at a high rate.