I believe bigmouth_strikes is correct on this. The hydroplanes do not generate lift as such, but they are used to alter the aspect of the submarine and to aid it in changing depth, along with the balast tanks. As far as I can remember, what the submarine will usually do is make itself attain neutral buoyancy, and then use the hydroplanes, very much like an aircraft would use it's elevators, to change the attitude of the sub. This will cause the submarine to go up and down. The ballast tanks can be used on their own though, to make the sub sink or rise (therefore making them useful in an emergency; pump lots of gas into the ballast tanks and you're almost certain to rise), whereas the hydroplanes need the submarine to be moving through the water horizonally to operate.