The outgoing breath (1); descending energy in the astral (energetic, as opposed to physical) spine. (2)
According to Swami Kriyananda, it is the flow of prana (rising energy in the astral spine) and apana (descending energy in the astral spine) that prompts the lungs to breathe. (3)
For more explanation of energy and the astral spine, see The Art and Science of Raja Yoga by Swami Kriyananda.