There is a law of physics which tells us that an increase in atmospheric pressure allows for more gas to be dissolved into any given liquid. This law is displayed in every can of soda. When soda is canned at the factory, it is put under pressure so that more carbon dioxide can be dissolved into the soda causing it to be "carbonated." When the can is opened, the pressure is released. Without the pressure to keep the gas dissolved, the soda loses the carbonation.
This same principal applies to oxygen and plasma (the liquid component of blood). When the body is under pressure, plasma, cerebral fluid (fluid in the brain), and other bodily fluids will dissolve much more oxygen than they would if they were not under pressure. Dissolved oxygen is much, much more easily moved throughout the body. Not only does this increase the amount of available oxygen to the tissues, it pushes this dissolved oxygen to reach areas of the body and brain that would normally be difficult to reach, such as areas of impaired circulation or damaged brain cells.