Retention of the breath
Holding the breath
The cycle of breathing may consciously be stopped, temporarily. This is called a breathing retention. One can retain the breath after an inhale, after an exhale or midway.
When holding the breath, oxygen (O2) in the lungs is exchanged with (CO2) from the blood. If the next inhale or exhale is delayed, after some time, (CO2) level in the blood starts to rise and at a certain point, air-hunger is felt. If the breath is held further, the rising (CO2) level in the blood, causes a trigger and air-hunger is felt. If the breath is held further, an unpleasant feeling arouses, followed by fluttering of the diaphragm. The body is signaling, to hurry up and complete the next cycle of breathing. If the breath is held further, the feeling of suffocation starts. The body is signaling that it is a matter life or death, emergency.
The human body does not have a sensor for the level of oxygen (O2) in the blood. It does have a sensor for the level of carbon dioxide (CO2). What triggers air-hunger, is carbon dioxide (CO2) in the blood. Different people have a different tolerance for rising (CO2) level in the blood.
Some people believe that denying food from the body, for a limited amount of time (fasting) has health benefit. In the same way, there are some people who believe that denying oxygen from the body, for a limited amount of time (air shortage), has health benefits.
Allowing (CO2) levels to rise in the blood and then suddenly allowing oxygen-exchange by completing the breathing cycle, works a bit like opening a clogged draining-system, by administrating sudden pressure.