Hyperbaric chambers are usually sleek cylinders with long observation windows. Inside there's a place to lie down, and often a mask that goes over the patient's face. Once the patient is inside, the chamber is sealed and filled up with pure oxygen. In fact, so much oxygen is pumped in that the hyperbaric chamber winds up at several times atmospheric pressure.
These scifi-looking devices are used for wound treatment, or carbon monoxide poisoning, or when some celebrity wants to live forever. Sometimes, they're used for treating flesh-eating bacteria. Then it's a case of a scifi disease meeting a scifi contraption.
These chambers create the kind of pressure that divers feel when they go deep enough underwater, and it's intended to have the same effect on patients that it does on divers. It's meant to allow more gas to force its way into the body. When more pressure is on liquid, more gas and other particles can dissolve inside it. It's only when the pressure is released that the gas bubbles out. In champagne and soda, carbon dioxide is forced into the liquid. In the case of deep sea divers, the air in their tanks dissolves into their blood, forcing them to come to the surface slowly if they don't want their blood to fizz like cherry coke. When someone is in a hyperbaric chamber, oxygen is forced into their blood stream. The intense pressure of oxygen on the rest of the body also exposes more tissues to this necessary gas.
This surge in oxygen can really give the body a boost. It promotes healing in deep burns or severe wounds. It's also used to treat carbon monoxide poisoning. Carbon monoxide tends to combine with hemoglobin in the blood and keep it from carrying around any oxygen. High pressure oxygen can increase the amount of oxygen being carried by the blood, preventing the supply of oxygen to the body from being choked off. It also reduces the amount of time that carbon monoxide can hold on to the hemoglobin.
Flesh eating bacteria is any of a group of different kinds of bacteria which tend to get into people who have open wounds around raw meat, bleed in the ocean, or hang out in hospitals (those who find that comparison frightening can reassure themselves that at least they won't get eaten by sharks in a hospital, so it's marginally safer). The bacteria get into soft tissue and spread like crazy, dissolving the tissue as they go. Many of them tend to be anaerobic bacteria – bacteria that live without oxygen. Oxygen can be toxic to anaerobic organisms. As they turn the body to mush, they cut off blood flow, further reducing oxygen to that part of the body. This behavior kills the body's cells and makes a nice home for the bacteria. The hyperbaric chamber reverses this process. Whether they make contact with the air, or make contact with bodily fluids, suddenly the bacteria are inundated with oxygen, killing them off. Hyperbaric stays alone don't kill off all the bacteria – most people lose some flesh to surgery and get a lot of antibiotics - but it does help pick off the last of the infection.
Lethal as it is for flesh eating bacteria, for the person inside the tube, the hyperbaric chamber is pretty mild form of treatment. The most unpleasant aspect that people complain of is a slight pressure on the ears inside the chamber and lightheadedness afterwards. Of course, since they are lying in a tube full of pure oxygen, a spark could blow them up at any moment. But that's not a big concern when you've got flesh-eating bacteria.