The dropper that functions as the diver is partially filled with water so its bulb just floats. When pressure is increased in the bottle while temperature remains constant, the volume of the air bubble in the dropper is reduced.
Additional water enters the dropper, the dropper's buoyancy decreases, and it sinks.
When the pressure decreases, the process is reversed, and the diver floats to the top of the bottle again. When temperature is decreased while pressure remains constant, the volume of the air bubble in the dropper is decreased. The dropper's buoyancy decreases, and it sinks. When the temperature increases, the process is reversed. Cartesian divers are a quick and simple way to illustrate relationships among pressure, volume, temperature, and buoyancy.