Breathing is an exceptional function of the body in that it is ordinarily regulated automatically by the Autonomic Nervous System, but can be consciously modified. Because of this, it can act as a doorway between the conscious and unconscious aspects of the self. Of course, the yogic tradition claims that all functions of the body controlled by the Autonomic Nervous System can, with practice, become volitional—even the beating of the heart. But until the yogi attains that level, practicing control of the breath is the most accessible way to create a bridge.
In order to guide your students along this path, it is helpful to have some understanding of the basic physiological functioning of the breath. Here's how the body is affected by it: As we inhale, the contracting diaphragm (the primary respiratory muscle, which is like the skin of a drum separating the thoracic cavity from the abdominal cavity) descends on the organs below, creating pressure. As a result, the thoracic cavity expands and the abdominal cavity contracts somewhat. As we exhale, the opposite occurs: the diaphragm relaxes and releases upward as the ribcage relaxes inward, allowing for a counter-intuitive spaciousness in the abdomen. This feeling of space in the abdomen can be difficult to feel in an individual with any restriction in natural free breathing, but is easily measurable in infants. During deep prolonged inhalation, a pressure is created in the thoracic cavity that stimulates several effects of the Sympathetic Nervous system (the branch Autonomic Nervous System that creates the "fight or flight response"), the most notable of which are temporary increases in heart rate and blood pressure. Deep prolonged exhalation tends to activate the opposite branch of the Autonomic Nervous System--the Parasympathetic--which again has many effects, including the temporary--but immediate!--drop in both heart-rate and blood pressure.
This is easily felt: Sit quietly for a little while, consciously lengthening your breath as much as you comfortably can and allowing it to round out so that the inhalation flows directly into the exhalation. Once you have established rhythm of long smooth comfortable breathing, place two fingers to the side of your larynx and feel your pulse. If your breath is unforced and long, you should be able to measure the increase in your pulse as you inhale and the decrease as you exhale.
Yoga, in very general terms, is a practice of balancing opposites. Often in our practice and teaching we aim to balance the inhalation and exhalation, which has a neutralizing effect on the currents of the two branches of the Autonomic Nervous System. Depending on the desired effect, however, modifying the focus toward the inhalation or the exhalation will greatly shift the energetic result of a Yoga practice.
The inhalation, though often thought of as an expansive breath, actually creates pressure around the heart, which shifts the system--at least during the breath cycle—into the Sympathetic system. Deep exhalation tends to shift it in the other direction. Thus, in personal practice, if one has a tendency toward anxiety and is trying to release stress, a breath ratio that emphasizes exhalation will be more helpful. On the other hand, in an individual who tends towards depression or lethargy, the same breath ratio will reinforce these difficulties.
In the simplest terms, brief breath retentions at the end of the cycles of the breath (at the end of the inhalation or exhalation) will tend to reinforce the effect of the preceding breath. There are, of course, more subtle and complex ways of looking at the energetic effects of the four elements of the breathing cycle. For deeper information on the various pranas and how they relate to more subtle levels of practice see David Frawley's Yoga and Ayurveda and Swami Muktibodhananda's translation of the Hatha Yoga Pradipika. In any case, breath retentions are extremely powerful practices, and it is important to have a personal experience of what you are working with before beginning to teach them, especially because not everyone present may have the same needs. Modifying conscious breath rhythms outside of an equal ratio (where the inhalation precisely equals the exhalation) or using retentions can have extraordinary effects, and what may be beneficial for one practitioner on a particular day may be harmful in another body or at another time.
If you've decided that you're ready to use breath control to create distinct energetic effects in class, here's how to get started. Before class begins, evaluate the energy of the room. If your students are particularly fidgety and talkative and seem to have trouble settling into a yoga practice, it might be a good idea to try longer exhalations (or, for particularly seasoned students, very brief external retentions) right at the beginning of class. If you teach a Vinyasa-style practice, this can easily be done during the Sun Salutations by simply leaving a bit more time for the movements performed on exhalation or talking students through brief retentions at the end of each exhale, during which they hold each pose momentarily. In other forms of yoga, you can achieve the same effect by simply asking your students to sit or lay in meditation as they practice Ujjayi Pranayama (Victorious Breath). Whatever your style of teaching, if you take ten to fifteen minutes at the beginning of class to emphasize the exhalation (and perhaps add a retention at the end of each exhalation), you will notice a visible calming for the rest of class. The better you know your students, the more obvious this will be to you. It could even be profoundly startling to see students who constantly fidget just resting quietly, even in challenging poses!
Prolonging the inhalation, on the other hand, will tend to have an energizing effect. This is useful up to a point, but if it is overdone it can lead to a very noisy class, or even overload your students' systems with more energy than they know what to do with! A ratio of extended inhalation (possibly adding retentions after each inhalation) will tend to help with a class that seems fatigued, but it is important to observe, carefully, that the energy level of the class is in fact increasing as you teach the breath in this way. It will only work up to a point. There is a limit on how "energized" the body can get--though it can change throughout practice!--and it is important not to force the inhalation in a violent fashion. Such forcing creates anxiety and stress instead of the calm energy that is your goal. Ideally, make sure that your students are comfortable with complete exhalations before introducing a deeper ratio or retention in the inhalation, as it is through exhalation, regardless of the ratio, that excess is released. Even at a physiological level, the human respiratory system seems to place more emphasis on the removal of carbon dioxide than on the inspiration of oxygen!
Once you've practiced these tools yourself and experimented with using them in class, you'll realize that the breath can be as profound and powerful tool in shaping your class as any particular asana or sequence.
Jamie Lindsay has been teaching Hatha Yoga in various forms since 1996. He has studied with many senior Ashtanga teachers and spent two years in the Advanced Studies Program at the Iyengar Yoga Institute of San Francisco. The writings of the Bihar School of Yoga and the techniques of Univeral Yoga have been important influences on his studies, and his current teacher is Andrey Lappa.