"...Death consists of the passing out of the air. It is, therefore, necessary to restrain the breath". Hatha Yoga Pradipika, ancient Hatha Yoga manuscript. Artour RakhimovAs I was stopped at a red light the other day, an older man came jogging across the street in the crosswalk right in front of me. It was a huge intersection with 8 lanes or more across in both directions. The man was running with his mouth open and what is worse, he was blowing out hard (as if his life depended on it) with each breath. He had a look of agony on his face and I was afraid he wouldn't make it to the curb. I don't know if the blowing-out part was an intentional technique that he was using, but I do know he was hyperventilating pretty badly. Each out breath lost more of his lung's supply of CO2. His muscles were working hard and making more CO2, but unless his resting CP was 20 to 30s, he wasn't making up the balance and he was doing more harm than good. I wanted to honk my horn at him and yell, "Stop! Don't do that!"
At the other extreme, here is a YouTube video of Natalia Molchanova, a champion free-diver, going for 131m in Japan—with a breath-hold of about 3.5 minutes! Watch how she prepares herself before the dive by first taking several very deep breaths, and then just before hitting the water, she blows out quickly and forcefully several times. (Do not try this at home! Even Molchanova wouldn't do this without a spotter in the water with her in case she loses consciousness.)
She is hyperventilating on purpose, tricking her breathing center into thinking there is no need to breathe. She doesn't want her diaphragm to spasm too soon. By holding her breath, CO2 levels will rise again. Free-divers hold their breath after an inhale and are very much aware of the fact that it is CO2 levels that initiates breathing. Also notice what she does at the end of the dive (at about 5:24). She comes up, takes a few breaths and smiles! No panting, no big huge breaths. Her chest barely moves.
She wears a weight around her neck to minimize the amount of movement she has to make so she doesn't have to work to stay under the water. Your muscles make CO2 when you use them and she doesn't want to raise her levels too soon. Free divers can stay under water for 5.5s (or even more probably, I am no expert on the sport) if they don't move. By the way, the divers call this apnea, which is the correct meaning of the word, but it's a far cry from the apnea that tries to kill you in the night.
When you hold after an inhale, you can hold your breath longer and the time is related to your lung capacity and training. Holding after the exhale, as we do in Buteyko, is a measure of your CO2 levels. You do not have to have Molchanova's genes, or chest capacity, in order to increase your CP. Divers also talk about the serenity of their sport and that also is a function of CO2 stores. CO2 is a relaxant of smooth muscles (such as the diaphragm). There is also something called the diving reflex, a holdover from our evolutionary past which can be activated just by holding your breath and putting your face into cold water, as when you splash cold water on your face to relax. (It's interesting to note that seals hold their breath after the exhale.)
One image Rakhimov suggests you use to relax the diaphragm when doing a CP is to feel like you are diving into water. Just for the beauty of it, here is another video of Mulchanova (and her son) diving through the arch at Blue Hole in the Red Sea.