As pressure builds, it thrusts equally against alI water surfaces. But only one mass of water is free to move, thet in ouput tube, which is open at other end. The water, rising in hot chamber of displecer tube, pushes hot air toward cold chember where there is now more space. Air temperature drops and volume contracts, sucking water back through 0uput tube. This motion pushes cold air back to heated space, end cycle repeats.
Frequency is about one cycle per second.
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.