What makes a bathyscaphe rise in the water?

Dropping auxiliary ballast ( steel shot in some cases) or blowing the conventional ballast tanks- which is a slovwer process due to inertia at the lower depths, not a rapid- woosh! like a sub down, say 400 feet, which will Blow Negative at this depth, and break surface timed by a chronograph or stop watch 30 seconds later- the rate of ascent is 800 Feet per minute! not so with the Deep Bathyscaphes. They come up slower.