The breathing/buoyancy bag is fitted with a non-return release valve which allows air to escape from the bag as the user ascends towards the surface and the water pressure decreases.
The usual Royal Navy DSEA rig also included an emergency buoyancy bag on the front of the main breathing/buoyancy bag to help keep the wearer afloat after reaching the surface even if he had exhausted the air in the breathing/buoyancy bag.