Prana, the first, is respiration. Apana, the second, is a help to excretion; it helps to cleanse the body of all excreta. The bowel movement comes from apana, and if you know how to work on it, you can cleanse your bowels as nobody else can. Yogis have the cleanest bowels. And that is very, very meaningful because once the bowel is totally clean, once your intestines are perfectly clean; your whole being feels light, as if you can fly. The burden disappears.
Ordinarily your intestines carry much rotten excreta – your whole life’s, layers upon layers. Inside the inner walls of the intestine, excreta goes on accumulating dry and hard. It creates many poisons; it makes you heavy. It makes you more available to gravitation. Much emphasis is given in yoga to cleanse the whole stomach so that no toxins remain in it, because they go on circulating in your blood, they go on circulating in your brain, and they create a particular energy field around you which is heavy, dark, black.
When the intestines are perfectly clean and clear, the aura arises around your head; and people who have perceptive eyes can see it very easily. And you feel as if you are like a feather.
The second, apana, is excretion. The third, samana, is digestion and providing body heat. If you know the function of the third and if you become aware where it is, your digestion will become absolutely perfect. Ordinarily you eat much but you don’t digest it. You go on eating and you never feel satisfied; you go on stuffing the body without digesting it. If one knows how to use samana, a small quantity of food will give him more energy than a big quantity of food ever gives to you. That’s why yogis can fast for many days without any harm to their body. Once in a while they will take a little quantity of food, and they will digest it totally. Your food is not totally digested. That’s why your excreta can become food for some other animal and he can digest it; much food value is still left in it.
Osho, Secrets of Yoga, Chapter 9: "To Know What Is".