The front of the Liberty is dominated by a set of breathing-loop hoses and counterlungs, though for the purpose of explaining the functionalities, it is best to start with a less obvious component – the dive-surface valve (1).
The dive-surface valve (DSV) looks simple, but appearances can be deceiving, which becomes apparent immediately upon the first exhalation into the rebreather. The check valve in the DSV ensures that exhaled air flows in the direction of the arrow via the breathing-loop hose to the T-piece (2a). The T-piece is connected directly to the counterlung, which is essentially nothing more than a bag made of impermeable material. The counterlung starts to be inflated with exhaled air, operating in the exact opposite rhythm of your lungs, hence its name.
When the counterlung is full, the air continues further in the direction of the arrow to the air-mass treatment unit (4), where it passes through a filter that absorbs the exhaled carbon dioxide, is replenished with oxygen and continues according to the second arrow to the other T-piece (2b) and fills the other counterlung (2b).
Now we arrive at the point when your lungs are empty and the counterlungs are full. This is an important difference from open circuit. The volume of air has not changed; no bubbles of precious air have escaped into the water, whereas at this moment a diver using an open-circuit apparatus would be trailed by a cloud of bubbles.
Exhalation is followed by inhalation. First we inhale fresh air from counterlung 3b (another check valve in the DSV takes care of the flow direction) and then also air from counterlung 3a via the treatment unit.
The circuit is closed; instead of several liters of air from the tank for each inhalation, you have consumed only a miniscule amount of oxygen. You can spend hours in the water regardless of depth; you can photograph fish without exhaling bubbles or attach a bomb to the bottom of an enemy ship right under the noses of the watchmen on deck.