Retention of the breath
We can make a conscience choice and hold our breath, that is stopping the cycle of breathing temporarily. This is a breathing retention. We may also make a retention midway, while the upper part of the lungs full or only the bottom part (the stomach) full.
When we hold our breath, the oxygen (O2) in our lungs exchanges places with the (CO2) in the blood. If we delay the next inhale or exhale, after some time, (CO2) level in our blood starts to rise. As a result, we start to feel air shortage, which leads to an unpleasant feeling. Our body is signaling to us that we should hurry up and complete the next cycle of breathing. If we further continue to hold our breath, we start to feel suffocation, a very unpleasant feeling. Our body is signaling us that it is a matter life or death, emergency.
Our body does not have a sensor for the levels of oxygen (O2) in the blood. It does have a sensor for the level of carbon dioxide (CO2). What triggers our air-shortage-discomfort, is carbon dioxide (CO2) in the blood. We all have a different tolerance for rising (CO2) level in the blood.
Some people believe that denying food from their body, for a limited amount of time (fasting) has health benefit. 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.