In an article in Astadala Yogamala Volume 6, Mr. Iyengar writes about the body being like an instrument. He says the body is like a piano or a violin, or any other instrument and each and every part of it needs to be tuned.
"The nerves in our body are like the strings, the brain is the intellectual part which is similar to the sound production from the instrument. The nose is the bridge of any instrument through which the breath is adjusted at every stage of the asana (posture). The spinal vertebrae are the knobs of the instrument, so I have to tune the knobs of the spine to such an extent that the nervous system, which starts from the spine, do not over-extend or under-extend. If there is under-extension the sound does not come, if is is too tight the strings snaps. So, one has to keep the exact pressure so that the tuning of the body takes place in such a way that one can listen to the inner vibration of the sound of the body. As far as I am concerned, I hear the sound, the vibration, and adjust to get equilibrium in body, mind and self while in asana, pranayama and dhyana."Namaste.