Inside this elbow is a poppet valve that is held "up" by the water pressure found in theis reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon.
Inside this elbow is a poppet valve that is held "up" by the water pressure found in the system, closing the air entrance to the device. If the pressure in the "upstream side" is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system, breaking the siphon.