Recirculation lines keep hot water flowing to the farthest point of a plumbing system so the person farthest away from the water heater doesn't have to wait long periods for hot water. Otherwise, the water in the plumbing would cool off. At peak periods, however, it is possible for so much demand for hot water to develop that cold water will be drawn off the bottom of the tank and backward through the recirc line, causing people taking showers to get cold flashes.

Swing check valves are one option to prevent that. They have a door inside that opens to permit recirc water to return, but shut when water tries to flow the other way.

They can be identified from spring check valves because they have a nut on top, visible in the photo, that a spring valve lacks.