”Asanas are like various branches spreading in different directions. Pranayama, which aerates the body with energy, is like the leaves which aerate the entire tree.”(Light on Pranayama, chapter 2, p.6)
As the quote says asanas lay the foundation, make the pathways for a pranayama practice. On a muscular and organic level the body is opened up through the practice of asanas. Pathways are cleared of physical and mental impurities bringing us closer to a stable mental and emotional state. Through asana practice we develop clarity of mind and stillness of mind, Pratyahar, (the nervous system is quietened) which are essential for the practice of pranayama. We also develop concentration, Dharana, and are able to take ourselves deep within which is an essential quality for Pranayama practice. We would otherwise not be be able to sit for a lengthy period for pranayama without becoming unstable and distracted on a physical and mental level.
The quote also highlights that pranayama practice will feed the asana practice, the branches will not grow without the leaves bringing vital energy to the tree. When an asana comes more easily, a connectivness between the physical and mental body is felt, thus opening pathways for the prana to flow more freely through the body.
Looking at the meaning of the word pranayama where prana means life force or vital energy and ayama means ascension, expansion and extension the direct link to asanas can be seen. The asanas work directly on the spine and surrounding muscles to lengthen, strengthen, stabilize and release tension. They also prepare the chest by doing a broad range of asanas that will extend, broaden, open the chest which then has the corresponding effect on the lungs. This is vital preparation for pranayama where we learn to control the breath without creating stress and tension in the body and mind.