The autonomic nervous system is divided into the sympathetic system, which is often identified with the fight-or-flight response, and the parasympathetic, which is identified with what's been called the relaxation response. When you do yoga - the deep breathing, the stretching, the movements that release muscle tension, the relaxed focus on being present in your body - you initiate a process that turns the fight or flight system off and the relaxation response on. That has a dramatic effect on the body. The heartbeat slows, respiration decreases, blood pressure decreases. The body seizes this chance to turn on the healing mechanisms.