When pressure in the cold water supply drops, the piston moves to the cold end, reducing the opening for the hot water supply. Temperature remains constant. When pressure in the hot water supply drops, the piston moves toward the hot end, reducing the opening for the cold water supply. Again, Temperature remains constant. In the event of a cold water supply failure, the piston moves to the extreme position on the cold end, which cuts the hot water supply completely.
All water flow through the valve is shut off.