Inline Shut-Off for Second Stage
Generally this shutoff is used for the purposes of isolating the Automatic Diluent Valve (ADV), Oxygen Solenoid, or manual add valves (MAV) on a rebreather. The benefits of being able to isolate these systems are many. In the event of a free flow divers can easily shut off and isolate gas addition systems and end their dive in a reduced but manageable state.
The isolation of an ADV is also beneficial during hypoxic trimix rebreather dives where the gas in a diluent cylinder may not be breathable as shallow depths where inadvertently adding it could result in the rebreather diver losing consciousness.