The Energetic Effects of Pranayama
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 pPranayama (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!