The air goes through its cyclical change in volume as it is forced from hot to cold chamber and back again by sloshing of water in the displacer tube. As water in displacer tube sloshes to the right. Air is shunted into the cold chamber where tempature falls and volume contcacts. This sucks water up through the bottom ball valve. Eventually, gravity begins to restore the balance in a pendulum-like effect.
Water sloshes back to the cold chamber.
This forces the air to the hot side where it is heated end expands, forcing water out of the top ball valve. The body of a flask functions as both fluid reservoir and part of displacer. The pumping impulses suck up water at suction inlet end squirt it out outlet tube. Photo inset shows working lab model. Dimensions of all water-filled components are critical. and entire unit must be finely tuned to sustain oscillation.