The stern planes control the angle of the submarine at all times. The rudder of course, controls the direction. The bow planes or fairwater planes (when mounted on a sail) control the depth. It doesn't make sense, until you understand "hovering." Like a helicopter, there are certain things a sub must do that involve hovering. The stern planes control the attitude or angle of the boat and are affected by movement of fluid through out tanks, folks moving from one section (forward to aft) to another, etc.
The physical amount of depth the sub must be at, will be determined by the bow/fairwater planes. So once the sub reaches say, 80 feet - periscope depth - the stern planes will keep the sub at that depth with the bow planes used to keep tweaks on the depth. It's a science and an art and a good/competent diving officer is the captain's right hand man in the overall chain of command, outranking even the XO.
"Dive" is a coveted title. The normal control party will consist of (diesel boat) Officer Of The Deck (OOD)(officer), Quartermaster Of The Watch (QMOW)(enlisted), Chief Of The Watch (COW)(enlisted), Dive (officer or enlisted), Fire Control Technician Of The Watch (FTOW)(enlisted) helmsman, stern planesman and bow planesman. On a nuke boat, the helmsman does the bow/fairwater planes.
Staying at periscope depth is a fine art that literally determines the life of the sub. Come up too high (broach) and you give your location away to even the most primitive sensors, let alone MK-1 eyeball. Keep yourself too low (dunk) and you can't see your target accurately. So a captain will normally keep a cadre of exceptional dives in his mind and once you prove yourself, your chances of doing anything BUT diving officer will be slim. Keeping everything level ("in trim") is a dive's job and everyone (officer and enlisted alike) do what he says.
A good dive can keep the boat just right for a perfect silent attack or spying on a hostile shore or recovering special forces. A good dive knows how much water to pump/flood to compensate for shift change (when folks forward go aft to relieve the watch) and to compensate for water being used during showers/dish washing. It's an art, like I said and a good dive will leave a boat with a ton of medals and recommendations.