Over the years, I have come to understand that to get the physical and pranic effects that I want (for myself and my students) out of the pose, it is necessary to prioritise minimising any distortion to the shape of the spine. In other words, to come into the pose in a way that maintains the natural lumbar lordosis, or hollow, and thoracic kyphosis, or broad upper back-just as in a natural standing position.
The physical benefits of this include keeping the neck relaxed, and the shoulders un-tensed, and in maintaining free movement of the diaphragm so that breathing can be full and easy. Having a full and easy breath is even more beneficial to the physiology as a whole if there is uncompromised movement of the spine around the breath, with none of the muscles around the spine, or the spinal discs being overly tensed or compressed.
As for the pranic benefits, maintaining the natural curves of the spine and a full and easy breath means that we are getting the effect of improving and balancing both prana and apana with each breathing cycle.
The inhalation, and the area from navel to throat (the thoracic area) relate to prana. On the inhalation, from it’s natural kyphosis, (broad and slightly rounded at the back) the thoracic spine slightly extends (flattens or hollows). The heart centre lifts and moves towards the chin (resulting in a subtle jalandhara bandha at the top of the inhalation.) The back of the chest also being broad and open is important here to expand the prana area all around in three dimensions around the spine and not just through the front chest. All of this leads to an increased level of and greater effect from prana, the energy involved with vitality and vigour, and courage.
The exhalation, and the area of the lower abdomen to the pelvic floor (the lumbar area) relate to apana. On the exhalation, from it?s natural lordosis (hollow at the back) the lumbar spine slightly flexes (moves towards flat). The navel moves in and upward towards the heart (resulting in a subtle uddiyana bandha and mula bandha at the end of the exhalation). All of this leads to an increased effect of apana, the energy involved with elimination, release, and surrender.
(Downward Dog is in fact a whole body mudra and is often used in classical hatha yoga as a way to teach the bandhas).