The second part of the training is to practise escaping in a swimming pool. The trainee sits in a model helicopter (called a HUET) as it is lowered into the pool. As it submerges, they start using their rebreather and then escape through the nearest exit when the HUET comes to rest. This training is pretty daunting!
Just before they submerge, the person breathes out into the bag. They will then rebreath this air for the next minute or so. They can do this because exhaled air still contains about 80% of the original oxygen.
In addition to the rebreathed air, they can release a canister of compressed air into the bag, giving them another minute's worth.