Pranayama, in short, is conscious breathing as it is done in Yoga. It can be regulated in some way, usually by making it long and subtle in various ways or not regulated at all, just observed. It is usually regulated by interrupting it through holding the breath at the end of inhale or exhale or both, or anyway in the breathing process. The primary goal is soothing the mind, preparing it for meditation. The secondary goal is securing good health, boosting vitality and achieving longevity.
The capacity to do pranayama is developed slowly over time so long sessions of pranayama (15 minutes and longer) are not recommended for beginners. They should have short sessions up to 12 breaths per session and only gradually introduce retentions. More experienced practitioners can do twice as much (24 breaths per session) and hold their breaths longer. The maximum of 36 breaths per session with long holds can be useful only to very experienced practitioners, and anything more than that can hardly be useful to anybody. A few shorter sessions of pranayama done throughout the day are more useful to most practitioners than one long practice.
Also, an appropriate asana practice (moving the body in close coordination with the breath so that the breath initiates and envelops each movement) is a must before pranayama as an adequate preparation for it. And, if a person breathes well during asana practice, there is usually no need to do much of pranayama, they are one and the same practice. Therefore, divorcing asana from pranayama is missing the point of both.
Ujjayi (producing a hissing sound in the throat while breathing deeply) and nadi–shodhana (alternate nostril breathing) pranayamas are the ones that are learnt first and can be done daily for longer periods of time. Almost all other pranayamas are done occasionally or temporarily for specific purposes, usually as therapy. And, most importantly, all pranayamas must be done very gradually and under a close supervision of a competent teacher. Otherwise, there is a great chance that something will go wrong or no results achieved.
Also, pranayama, or conscious yogic breathing, was and is practiced differently according to the context in which it is used and depending on the purpose for which it is used. And it should always be adapted to the individual and preceded by an appropriate practice of asana understood and practiced primarily as a moving pranayama preparing for pranayama proper, usually done in a static seated posture with the legs crossed. The eyes should be closed and the whole attention given to the movement of the breath being carefully followed and quality of the mind being progressively soothed.
Although originally it might have been practiced as psychosomatic purification or atonement for past sins, it soon evolved into a method for soothing the mind and later on even into a preventive/therapeutic practice. But, essentially, it is a devotional practice as the whole of Yoga: giving oneself totally to that which inspires us. In this case literary to the breath that literary in-spires us with life and with spirit.
In hatha–yoga, pranayama/prana-samrodha is therefore the central practice because respiration is the most critical function of the human organism, even more critical than the heartbeat. Nadi-shodhana (alternate nostril breathing) and ujjayi-pranayama (throat breathing) are considered to be the most significant ones, the first one being very old and predating hatha-yoga, and the second one being relatively new and invented by hatha-yogis.
When combined into three distinct pranayamas – anuloma-ujjayi (inhaling ujjayi and exhaling through one nostril), viloma-ujjayi (inhaling through one nostril and exhaling ujjayi) and pratiloma-ujjayi-pranayama (combining anuloma and viloma into one practice) – their benefits are enhanced and deepened. Unlike nadi-shodhana and ujjayi that mostly work on the mental plane calming the mind, these three pranayamas affect our emotional core and help us deal with our emotions more creatively.
In advanced pranayama practices involving long retentions of the breath (kumbhaka), various breath ratios (vrtti), the application of the energy seals (bandha-mudra), creative visualization (bhavana) and the silent repetition of appropriate mantras (mantra-japa) the idea is to improve concentration and raise the intensity of involvement into the process of being alive and going ever deeper into ourselves to know ourselves. Then meditation, or dhyana, arises spontaneously providing intuitive insight into Life that we altogether are. And all the knowledge and power that goes with it is then appropriately used for the well-being of all beings and the benefit of all humanity.
The right hand forming mrgi-mudra is usually used to control the flow of the air through the nostrils and left hand rests on the left knee with the palm turned upward counting the breaths by touching the 12 knuckles of the fingers with the thumb. That is the reason why one round of pranayama is usually considered to consist of 12 breaths. One such round is considered enough for a beginner. The advantage of counting the breaths by touching the fingers instead of mentally counting them is precisely eliminating every mental exertion.
At a higher level of doing pranayama, no counting is necessary; it is spontaneously felt when it is appropriate for us to stop doing it. The right measure is the key to success and the immediate indication that our practice of pranayama was beneficial for us is the sensation of lightness spreading throughout the whole body, the sense of peace pervading the mind and the feeling of fullness expanding in the Heart. Then meditation simply happens as abiding freely in our natural state, which is powerful peace and peaceful power of a life lived straight from the Heart, for the Heart.