Being a yoga practitioner, Dr. Singh has always been curious to know of how yoga works. First, after going through the book—Nature and Health, he could find that carbon dioxide stimulates vagus nerve. Thereafter he could also find a study that shows of how increased blood concentration of carbon dioxide (hypercapnia) in conscious humans can stimulate the vagus nerve that in turn, manifests as respiratory sinus arrhythmia (RSA). Even more important, the results of this study also prove that when hypercapnia is coupled with exhalation phase, a synergistic effect occurs that compounds the cardio-inhibitory response of vagus nerve. In some review articles, it was stated that increased blood CO2 concentration is the most porobable mechanism that determines an enhanced effect on the cardio-inhibitory response of vagus nerve during exhalation phase of the breathing rhythm. In another very interesting study, the importance of exhalation phase of the breathing rhythm in superficial acupuncture stimulation was revealed. As the effect is comparatively pronounced when acupuncture stimulation is applied only during exhalation versus continuous stimulation during both inhalation as well as inhalation phase. Since the above research studies clearly show of how “hypercapnia” and “exhalation phase” are the two most important determining factors for the stimulation of vagus nerve. Hence these research findings prompted the author to write this article that may further help yoga practitioners in controlling these important variables (blood concentration of carbon dioxide and exhalation phase) in an effective and efficient manner, on a moment-to-moment basis. The author, in the sub-head “Insights”, describes about the importance of Carbon Dioxide and Exhalation Phase in stimulating the vagus nerve. Furthermore, he also explains in details about the two most important breathing exercises (Pranayama and Buteyko method) in the sub-head titled, “Hypercapnic Exercises”.