Sri Aurobindo translates Chhandogya Upanishad, Chapter One, Section Two, Verses 7-9: “Then the Gods worshipped OM as this which is Breath in the mouth and the Demons rushing against it dashed themselves to pieces; as when an object striketh against firm and solid rock, it dasheth to pieces upon the rock. And even as an object hurling against firm and solid rock dasheth itself to pieces, so he hurleth himself upon destruction whose desireth evil against the Knower or whose doeth him hurt; for the Knower is as that firm and solid rock. With this Breath one cogniseth neither sweet scent nor ill odour, for it hath flung Evil from it. Whatsoever one eateth with this or drinketh, thereby it cherisheth the other breaths. At the end and last when he findeth not the breath, the Spirit goeth out from the body; verily he openeth wide the mouth as he goeth.”
The “breath in the mouth” symbolically represents Prana, the original energy of the Eternal that manifests the universe. While the breath in the nostrils is one of the subsidiary sense functions and thus, can experience both “good” and “bad” scents, the breath in the mouth is independent of this sense function. The Eternal, worshipped as OM, in the form of its energy of manifestation is impervious to the action of fragmentation, division, individual desire which is what underlies the action of the demons called “evil” and thus, represents the true fulfillment of the aspiration of the higher forces in the evolutionary creation.
When this “breath” is present, there is life in the body and when it departs, then the body dies and disintegrates back into its constituent elements.
The “Knower” of this breath resides in the status of the Brahman and thus is not able to be misled or drawn out from his knowledge of the Eternal; thus making him impervious to the action of the demonic energies of the fragmented view of the lower nature seeking its own self-aggrandisement as if it is separate from the Oneness of the universal creation.
Sri Aurobindo, The Upanishads, Chhandogya Upanishad, pp.349-366