Breathing is what it's all about. It can help if we think of breath, not as a noun, but as the verb: to breathe. Without the oscillation that is the tantamount feature of the process, there is no process. Life itself is not a thing, but an event. When you sit and hold your breath, you are still breathing. You are merely holding it in or out for a longer than usual spell. Some yogis and some Buddhist monks have held their breath for long periods of time. There are rumors about various mystics who have suspended their breathing for hours or days, surviving being buried alive, and so on. I read a suggestion that perhaps these fellows were only breathing very, very slowly. I don't know. Breathing exercise, as its name suggests, is not natural breathing. Like doing push-ups or chin-ups, running, swimming or walking laps, lifting weights or practicing martial arts, it involves an effort to do more, to push one's capacity beyond whatever are its boundaries. How to breathe is important. It involves three changes in the body: expansion of the abdomen, chest expansion and raising the shoulders. During Pranayama, we focus on each of those areas, beginning the breath by relaxing the abdominal muscles allowing an inflow of air. As air is entering the lungs we expand our chest. When the lungs are close to full we raise our shoulders a little to allow in as much air as we can. While exhaling, we relax our shoulders and chests, and we add to the natural outward pressure by contracting our abdominal muscles so as to drive out every last bit of air.
In The Complete Illustrated Book of Yoga by Swami Vishnu-devananda (Paperback - April 18, 1995), he suggests that we find a personal instructor if we wish to get deep into Pranayama, and I am passing that advice along to you. I claim no competence in the instruction of Yogic Breath Control.
I do know that we have to breathe, and that breathing can be a distraction. Professor Wood says that Patanjali does not recommend the prolonged and complex breathing rituals of Hatha Yoga. Certainly, Pranayama is one of the 8 limbs of Raja Yoga. In Aphorism II, 49 & 50, he acknowledges that, once a steady and pleasurable seat is obtained, the next steps deal with breathing. "The condition of the breath as outgoing, incoming or standing still, is regulated as to place, time and number, and becomes lengthy and fine." However, he goes on to say, in II, 51, "A fourth (condition arises) which casts aside the business of external and internal (breathing)." Alan Watts, in his discussions of Zen meditation and breathing, speaks of this same transcendence. He suggests watching the inflow and the outflow, not trying to do this or that, until we eventually lose track of the breath altogether.
In the sequence of Asanas that begins with the Sun Salutation, attention is paid to the ins and outs of breathing. In all positions that require arching backward, such as the Cobra, the student inhales. While bending forward in an asana such as the Plow, he exhales. Positions such as the Headstand or the Shoulder stand that require one to maintain the pose for some time can be measured by counting complete breaths. Following each asana, the student is advised to spend a short spell in a comfortable pose, relaxing prior to the next exercise. This time can also be passed by counting four breaths. When one has gone through the entire cycle, from Sun Salutation to Corpse posture, he will have breathed in and out with conscious attention hundreds of times. This explains much about how Hatha Yoga predisposes one to meditate.
Breathing has been a challenge for me since early childhood. I was born into a family with a history of asthma on each side. Both of my grandfathers finished their lives as virtual invalids due to the inability to breathe well. Parents, siblings and cousins have endured various levels of the same misery. I myself have been hospitalized more than once for this infirmity. Yet, during good times I am a long-distance hiker, a mountain climber, a body surfer, a swimmer and a student of Hatha Yoga. I honestly feel that, had I not pursued the discipline of Yoga, my lifetime of breathing would have been shorter and more tortured. I count every breath a blessing.
When I am not distracted by inhalation and exhalation, I am free to concentrate. Continued practice of Hatha Yoga will lead to this freedom.