No other concept there. Actually it`s a simple concept. Closing the valves you cut off the airflow and isolate the cargo compartment. After deploying the fire agent the agent/air mixture will keep constant and that will ensure that the fire will not ignite again. But as always there are some small leeks and the thruth is that the agent quantity will gradually decrease and also lose eficiency. That`s why you have a second bottle on ETOPS equipped airplanes with a meetering device that will keep on adding agent for the next 240 minutes (depends on the kind of bottle, ie time of ETOPS aproval) once activated, wich is usually 1 hr after the first bottle discharge.
hope that helps