Trap primer valve

Can someone explain how a trap primer valve operates & functions.

This was installed off the cold water line at a 2nd floor water tank.

Thank you

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When pressure changes in that line (a fixture is used) the trap allows a little water to pass into the trap, keeping it full of water and stopping any sewer gasses that could pass through a dry trap.

In my area its common to see the laundry floor drain being primed by one of the toilets. When the toilet is flushed water is sent through a check valve and down a pipe (in the slab) to the trap in the floor.