Water is siphoned from the tank into the back of the overflow, where it drains into a sump. Air bubbles tend to get trapped and accumulate in the chamber the airline is attached to, breaking the siphon. The pump sucks air out, maintaining the siphon and keeping the water flowing. Those Air Lifter pumps are notorious for failing rather quickly. A much better choice is a Maxi-jet 1200 power head w/ a venturi attachment. It is much more reliable and powerful.