Controlled Emergency Swimming Ascent (CESA) (aka "Free Ascent" or "blow and go") is a technique used by scuba divers as a safety measure when a diver has run out of breathing gas in shallow water and must return to the surface.
The technique involves simply ascending at a controlled pace, typically about 18 meters (60 feet) per minute, while exhaling slowly or vocalizing a continuous sound. As the diver ascends, the pressure in the lungs increases relative to surrounding water pressure. Exhaling serves to equalize this difference. Therefore, the diver can continue exhaling throughout the ascent and still have air in his lungs at the surface. For the same reason, if the diver fails to exhale during the ascent, lung over expansion injury is likely to occur. As the diver swims to the surface (making the "ahhhhh" sound), he will feel as if he is not running out of breath; this is because the compressed air he inhaled at depth is expanding as he ascends.
While in a practical sense there is little difference between a CESA and a "Free Ascent" (aka Emergency Swimming Ascent or ESA), the technical difference between the two is that in a CESA the regulator second stage is retained in the mouth and the diver exhales through it (in case gas becomes available due to the drop in ambient pressure) while in an ESA, the regulator is not retained and the diver exhales through pursed lips.
A free ascent is when a scuba diver with compressed gas in his system leaves his diving depth for the surface without his breathing apparatus. The recommended rate of ascent is 18 metres (60 ft) per minute. As the gases in the diver's lungs expand, they need to be exhaled rapidly enough to prevent over-inflation of the lungs. Free ascent is a technique that requires a great deal of training; military divers are sometimes trained in this technique. Some diver training organizations also include training for a free ascent (controlled emergency swimming ascent) in their syllabuses. Free ascent is an emergency procedure that involves risk but could save a diver's life from an otherwise hopeless situation.
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—Ralph Waldo Emerson (1803–1882)