These non-draining drippers are used in greenhouses that supply water from above. When the water supply turns off and the pressure reduces to 2.9 psi the dripper closes and then seals completely at 2.2 psi.
This keeps from having the entire overhead system draining out once the irrigation system is turned off.
The drippers will open up again when the system is turned on and the pressure rises above 4.3 psi.