Using a technique which was also employed in the iron-foundries of Italy as early as the the sixteenth century, water is made to enter a long vertical tube, usually with a funnel-shaped opening. In doing so, the water forms a gyre, and sucks air into the tube as well, forming an air-water mixture.
At the end of its descent through the tube, the water is separated from the mixture in a special container ("Aeolian chamber"), and may be used to drive a paddle-wheel, as in the case of this organ. The air is allowed to exit via another tube which is connected to the windchest. No bellows are involved - the pressure is regulated by controlling the flow of water into the system.