I believe they came about because of an FAA directive, any seat within a certain distance of a bulkhead must have them fitted on new type certificated aircraft or equipment such as new design seats, hence A346. They work from a box in the seat with a inertia switch which activates under high decelerations (ie crash !!), joining the buckle ends arms the system. I'm not entirely convinced by them to say the least, just waiting for the day when one goes off "accidentaly" in the face of a pax, watch the lawyers come running and the FAA running away.