on the 320, it 's automatically controlled (2 CPC cabin pressure controllers).
can override manually or close it (plus 2 safety valves in the back)for ditching like in new york!well they are not here for dichting, it s to relieve pressure if the valve stay closed.
so you have several choices:
automatic: you do nothing!
semi automatic: you set landing altitude in the computer.
manual: you maintain pressure on the switch(UP/DOWN) until you got cabin altitude required.
Ditching mode: close all valves(you don't want water to come inside the plane too fast), and bring me some fishes...
