A fire may either start in or spread inside structural components like walls, floors, ceilings, knee walls and other small spaces. These fires are almost always ventilation controlled and only burn in proportion to the oxygen available. The openings firefighters make when trying to cool hot fuel surfaces provide oxygen to the fire unless all the hot surfaces are doused with water through the opening. However, the opening will also create flowpaths allowing the fire to travel to other places through convective heat. The third problem is that the opening will prevent steam from filling the space to further suppress the fire.
Fognails provide a solution where cooling of the compartments can be achieved without making larger openings. The closed space denies oxygen to the fire, prevents flowpaths and traps steam to suppress the fire. After the compartments have been cooled and more resources have arrived, coordinated ventilation with further cooling of the surfaces can be achieved.