There are 3 described types of DUBB: Intentional hyperventilation—blowing off carbon dioxide before submerged swimming, thereby delaying central hypercarbic ventilatory responses Hypoxic training—extending capacity for underwater distance swimming or breath-holding Static apnea—breath-holding for as long as possible while submerged and motionless, including as a game In DUBBs, while submerged, hypoxia occurs first, followed by loss of consciousness (hypoxic blackout, breath-hold blackout) and then drowning.