Immersion circulators use pumps to move water through the heating element and back into the water bath. There are two basic approaches pumps can use. One way, called a reciprocating pump, uses a lateral motion to pull liquids through a space. A piston moving through a cylinder is an example of a reciprocating pump. As the piston moves out of the cylinder, it pulls in fluid. As the piston moves back into the cylinder, it pushes fluid out. With one-way valves, you can control where the fluid goes, redirecting the flow.
The other type of pump is called a rotary pump. Rotary pumps use a rotor called an impeller. It looks a lot like a fan enclosed in a pipe. If you submerge an impeller in water and then spin it, the spinning motion pushes fluids outward. If you use valves to create a one-way path, with water coming in from the bottom of a pipe and going out through the top, water circulates as long as the impeller continues to spin. The outgoing water creates a vacuum, which pulls more water into the rotary chamber.
In either case, the pumps circulate water by pulling liquid up from the bottom of the hot water bath and moving it to the top. As the hot water circulates, it heats more evenly than it would if the water were still. To understand why, we need to take a quick look at fluid dynamics.
Heat transfers through a fluid in a process called convection. In general, fluids are poor heat conductors but are good thermal insulators. When a fluid heats up, the particles within that fluid expand and become less dense. As the density of fluid particles decreases, they rise up and cooler, denser particles sink to take their place, creating little currents.
If you were to use a heating element on its own, these little currents of fluid would form naturally over time and distribute the heat. But it's a gradual process. The pumps circulate the water, creating forced convection, speeding up heat dispersal.