Answer: Rectal breathing is a technique for controlling the volume of air inhaled into the lungs by increasing the amount of air held in the rectum, thereby reducing the amount of air in the lungs. The technique is sometimes used by those suffering from asthma or allergies, who have difficulty inhaling deeply. It can also be used to reduce hyperventilation, as in anxiety attacks.
Rectal breathing is an ancient technique. It was described by the ancient Greek physician Galen in the second century CE, who wrote: "It is the best thing for a person who is affected by an asthma attack to make a fist and put his hand over his mouth and nose and then breathe through his rectum."
Rectal breathing is also sometimes used by competitive breath-hold divers, who can hold their breath for over 12 minutes. In order to reduce the amount of air in the lungs, and thus reduce the work of breathing, they hold a large volume of air in their rectum.