Excerpted from http://meditationformultitaskers.blogspot.com/
In my research with Dr. Ravinder Jerath, we found that meditation with breath control has demonstrably greater benefits than meditation with just mindfulness or visualization alone. In chapter two of our book, The Everything Guide to Meditation for Healthy Living, we explain the complex interplay between the brain, the heart, and the lungs. To put it simply, intentional deep breathing acts like a manual override on the body's fight-or-flight response. Taking a deep breath sends an "all is well" message to the brain, which then slows down the production of stress hormones. The heart rate may actually increase a little bit from deep breathing, but it's like the increase in heart rate that happens during exercise. I would not go so far as to say that meditation is the only exercise you need, but the effect is similar. The muscles that expand the rib cage during breathing--the diaphragm, the intercostals, and the abdominals--consume a lot of energy when they are deliberately activated. The yogic term for austerities--tapas--actually means "heat," and many meditators report feeling warm with the practice of deep breathing.
Pranayama or breath control can be incredibly complicated or incredibly simple. At its most basic level, breath control simply means breathing more deeply than usual. Most people take shallow breaths throughout the day, which leads to a diminished awareness of the abdominal region and a lower level of oxygen in the blood. This causes an over-identification with the thought processes, causing a furrowed-brow, brooding facial expression (and often an accompanying headache). Breathing deeply moves awareness down the spinal column to the heart center, which aids in slowing the frenetic pace of thinking and stimulating a broader form of awareness that situates the body within the surrounding space. To breathe deeply naturally leads to expanded awareness. One can reap most of the benefits of pranayama by simply taking deeper breaths throughout the day and especially during meditation.
My guru, Swami Satyananda Saraswati of Devi Mandir, teaches two forms of breath control derived from Rishi Kashyapa. During chanting, he recommends making the in-flowing breath equal to the out-flowing breath. A silent mantra is said while inhaling, and the scripture is chanted on the exhalation. For sitting meditation, he recommends the 1-4-2 ratio as taught by Kashyapa. This means inhaling for one, holding for four, and exhaling for two. Keep in mind that the count might be 4, 16, and 8 seconds or 8, 32, and 16 seconds, so long as the ratio is kept the same. There is no holding of the breath at the end of the exhalation, perhaps to circumvent the Hering-Breuer (inhalation) reflex. This technique leads to a wonderful stretch across the upper back and an increased awareness of the heart muscle. One can easily use the heart as a timer with a little bit of practice, counting the beats as they go by. One can also begin to isolate the abdominal, side, and chest portions of the inhalation.
Yoga classes will also teach alternate nostril breathing, which balances the left and right sides of the body. Most people have one nostril that just isn't as open as the other one, which can come from congestion or just physiological differences between the two sides. Most people do not have perfectly symmetrical nasal passageways without surgery. Alternate nostril breathing strengthens the "weaker" side. It can be done simply by using the 1-4-2 ratio with one nostril closed. The mudra used for closing the nostrils is the Vishnu mudra. This reminds one of the preserving function of God, for breath strengthens us both spiritually and physically.
Most people will find that they don't need any more techniques of pranayama than those listed here. More experienced practitioners will want to seek a qualified teacher before trying anything more strenuous.