Heat trap
The heat traps allow cold water to flow into the water heater tank, but prevent unwanted natural convection and heated water to flow out of the tank.
[1][2] Newer water heaters have built-in heat traps.
Many water-heating pieces of equipment have integral heat traps installed from the factory.
For water-heating equipment that does not already have factory installed heat traps, they must be purchased then installed in the inlet and outlet connections.
The heat trap stops this process, thus keeping the hot water inside the insulated storage tank.
Natural convection draws hot water into the pipe connected to the top and lets it recirculate into the container after it has cooled inside the pipe.
Right image:
The convective (micro)circulation is stopped by the loop which does not allow warm water to pass further into the pipe.