The built-in circulation pump generates a strong circulation current (upstream), that lowers the danger of blocking and channelling. A higher reaction rate is the consequence.
The in-coming water is de-oxygenated by the biological activity of the filter bacteria. If any or low oxygen is in the water the bacteria switch over to nitrate respiration and consume nitrate instead of oxygen. This de-nitrification process lowers the nitrate concentration of the water.
The out-coming water flows back to the aquarium with less nitrate.