On either side of the (astral or energetic, not physical) spine there is an energetic nerve channel, or nadi : ida on the left and pingala on the right. Energy automatically moves up the ida nadi, and down the pingala nadi, along with the breath.
When apana, or downward-moving energy, travels down through the pingala nadi, its movement is related to a negation of external, rather than of internal, conditions. This downward movement is accompanied by physical exhalation, which is why, when you feel negatively inclined toward things, you exhale more heavily than normally.
Eventually a yogi learn to interrupt this cycle, and as the energy reaches the base of the spine after traveling down the pingala, catches it and redirects it up into the deep spine or sushumna, bringing spiritual advancement. (1)
The folllowing video explains how to get control over the upward and downward movement of energy in the spine:
- ↩ The Art and Science of Raja Yoga, Swami Kriyananda. Step Twelve, “The Anatomy of Yoga.”