They connect to a cold water SUPPLY line running to a fixture - usually a sink or toilet. A clothes washer is also sometimes used. When water is used by the fixture, the resulting change in pressure within the line activates the trap primer and a small amount of water is discharged by the trap primer for the floor drain p-trap. You have to ensure the fixture demands enough flow in relation to the static pressure of the line the primer is connected to. They also need to stay accessable.
If there is no demand by the fixture they are connected to, the p-trap will dry up. The operation has nothing to do with the level of water in the trap. There are other solutions for a p-trap which rarely gets any water in them. Some floor drains use balls or openings that close up when dry. I have only used trap primers and they work well, if the parameters are met.
Here is the skinny on Zurn's unit - http://www.zurn.com/operations/specd...llation/t1.pdf