The great eighth-century yogi and philosopher Shankaracharya said, "Yoga asana is that in which meditation flows spontaneously and ceaselessly, not that which destroys happiness." In other words, when yoga poses are well aligned, the breath flows right up the front of the spine into the spacious radiance of the body's central axis. The experience is beautiful and sublime.
Realistically, our practices can rarely be called sublime. The mind and ego seem programmed to stay out of the central axis, making practice a superficial exercise in self-improvement rather than the precise observation of, and insight into, the nature of our bodies and minds.
An excellent way to counteract this tendency is to link the two basic internal patterns that control inhaling and exhaling. These are called prana (upward spreading breath) and apana (downward contracting breath). The prana controls inhaling; it is felt as an upward floating, branching, and flowering pattern. Its home is the core of the heart. The apana controls exhaling. It is the downward rooting flow, which contracts, or tones, into a seed point at the center of the pelvic floor. With each breath you take, prana and apana organize the movement of bones and muscles. Prana lengthens, or extends, the spine (as in a backbend) and brings the legs into internal rotation; apana rounds, or flexes, the spine (as in a forward bend) and rotates the legs externally.