Asana is steady comfortable posture
Patanjali Maharshi does not lay much stress on either asana or pranayama. They are developed in different branch called Hatha Yoga which details are talked about in Hatha Yoga Pradipika. Asana can be divided into mainly two divisions, meditative poses and poses for health and strength. There are four excellent meditative poses one can adopt for meditation, they are:
Without securing a steady asana, one is unable to do well in meditation, because the feeling of uncomfortable in the physical levels will effect the mind to concentrate. Therefore a steady and comfortable posture is one of the limbs which help one establish in meditation. Even in Bhagavad Gita 6-13, Lord Krsna gave instruction of meditation, "holding oneself firm without moving, keeping the body, head and neck in one straight line, (as though) looking at the tip of the nose (eye position) and looking in all direction." In BG 6-11 the seat itself is described," having arranged one's seat (made of) piece of cloth, a skin, and a grass mat layered in (reverse) order, in a clean place, firm, not too hight and not too low."
IV. Pranayama-breath control
Pranayama is a long duration or subtle according to the inhalation (puraka), exhalation (rechaka) and retention (kumbaka), regulated by place, time and numbers.
Prana can be said to be principle of energy/force. Itself is not energy, but energy is grosser manifestation of prana. Its is found in all forms, either in living being or non living being (the manifestation is different because of the medium as non living being can not reflect the suksma sarira (subtle body) where the prana located).
In Hatha Yoga Pradipika 2-2,"When the breath is unsteady, the mind is also unsteady. But when the breath is steady, the mind attains steadiness." The meaning of the breath here is not our physical breath, but Prana (vital air). The relationship between the mind and prana just like a tree and wind. We can not see the wind is blowing unless we refer to the motion of the tree. It is same with our thought. When the prana is unsteady, our mind is also disturbed, then one can not sit quietly for meditation. Therefore prana need to be regulated through physical breathing itself, because physical breath is external manifestation of gross prana.
Prana is absorbed through breathing, and the excess of prana is stored in the subtle nerve centre (nadi). The yogi stores abundance of prana by regular practice of pranayama, therefore their body is very vigour and their mind is very sharp.
A correct habit of breathing must be established by just doing simple diaphragm breathing. Most of the people even breath through the mouth, or breathing irregularly. One should start with diaphragm breathing with one's one pace. Then bring the breathing in a proportional way (1:4:2) inhalation 1 count ; retention 4 counts ; exhalation 2 counts. This ratio needs to be increased based on one's capacity.
There are quite numbers of pranayama exercise, but some of them are in the category of advance pranamaya where only suitable to practice in controlled environment with controlled diet. Some minor pranayama one can practice which also give abundance of benefits, they are: anulomaviloma pranayama, sitkari, sitali, and bhrahmari.
This principal energy is categorized by their functions: 1. Prāna, responsible for the function of respiration system.
2. Apāna, responsible for the function of elimination/excretion system.
3. Vyāna, responsible for the function of circulation system. 4. Udāna, responsible for the function of evacuation system. 5. Samāna, responsible for the function of digestion system.
They secondary prana like:
1. Naga, regulates burping.
2. Kurma, controls blinking.
3. Krikala, governs sneezing. 4. Devadatta, controls yawning. 5. Dhananjaya, controls the functioning of heart valves.