A diving buoyancy compensator or "stab" or BCD is required for the diver's equilibrium in the water.
The stabilizing jackets for diving make it possible to stay balanced at any depth. Many vest are offered, very complete with all possible options.
From the simple "club" jacket to the model incorporating removable lead pockets and a "tec" type harness, they all have an inflator.
The emptying of the vest will be done using fast high and low purges but also with the help of the inflator or a quick purge at the top of the corrugated pipe called Fenstop.
Note that it is not necessary to look for large volumes of BCD if you dive with a single bottle for example. The higher the volume, the more hydrodynamics will be reduced.