How does a gravel vacuum work?

Air is pumped into the side of an open ended tube one end of which is placed at the bottom of the tank. Air rises in water, so the rising air creats a collumn of rising water in the tube. A small material bag is placed at the top end of the tube and as the water flows into this bag it filters back into the tank. If the other end of the tube is moved around close to the mulm and rubbish on the (subtrata) bottom of the tank, the water flowing into the tube will lift the rubbish up the water column where it is filtered out and remains in the bag at the top for later removal.