With the laudable aim of making free-diving safer by eliminating the deficiencies and defects of the equipment available in America in the early 1950s, Barney Girden was committed, among other things, to creating a system for re-breathing one's own air.
It's unclear how it works, but it's an elastic rubber collar that inflates as you exhale. While you're at the surface, you breathe normally through the snorkel, but as soon as you descend, a valve closes and the exhaled air flows into the rubber bag, which inflates similarly to a frog's throat when it emits its familiar croaking sound.
Therefore, you can remain immersed beyond your apnea limit, rebreathing your own air for another apnea without surfacing. Note that this air is in perfect equipressure with the depth you are at, allowing you to inhale effortlessly and naturally.
At this point, however, some questions inevitably arise.
This air has already been used, so it has lost a significant percentage of oxygen and has gained carbon dioxide. It's difficult to make precise calculations, but the oxygen percentage will have dropped to around 16% from the 21% present in "clean" air, approaching the hypoxia risk zone, while the CO2 will have reached levels close to those of syncope.
A second breath of the same air might allow for a second, brief breath-hold, but it's risky even if you're just a few meters below the surface. Even if this invention by Frog's Throat was intended for emergencies only, something just doesn't add up.
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