When the lumbar spine is in an even arch in Viparita Karani, you will also feel that your ribs part to the sides a bit, creating more space for the organs of the belly and for your inhalations. Remember that this arch is positional. It is not achieved by actively tightening the erector muscles of the posterior spine, but rather by how you position yourself over the bolster. With the right arch, your tailbone will drop down, counterbalancing the weight of the thoracic spine moving in the opposite direction. When you are lying evenly, the sensation will be one of ease and openness without effort.
Finally, make sure that your face is parallel to the floor. Tuck your chin slightly, so that the extension of the thoracic and lumbar spines does not continue in the cervical spine (neck); this will enable you to enter an introspective state. Keeping the cervical spine in flexion will help stimulate the parasympathetic nervous system, the "rest and heal" nervous system, and thus create the feelings of calm that we all crave. Ending your practice or your day with this pose will not only mitigate the effects of stress but will also create a stronger immune system, a quieter mind, and a peaceful sense of self.
Judith Hanson Lasater, Ph.D., is a physical therapist. She recently completed writing Yoga Moves, a book about the anatomy and kinesiology of asana, which will be available in 2007. Her latest book, A Year of Living Your Yoga, has just been released.