The SBN controls buoyancy: push water out of a tank and the vehicle gets lighter and rises; let water in and it sinks. Trimmed to neutral, it hovers without spending propulsion energy.
For engineers
We use a pneumatic (compressed-gas) ballast rather than a pump-only design for the speed of a buoyancy change — important for an emergency ascent — and the design adds a mechanical drop weight as a passive failsafe. The drop weight isn't built or tested yet.