Sri Aurobindo translates Mundaka Upanishad, Chapter 2, Section 2, verse 4: “OM is the bow and the soul is the arrow, and That, even the Brahman, is spoken of as the target. That must be pierced with an unfaltering aim; one must be absorbed into That as an arrow is lost in its target.”
The Mundaka Upanishad provides a methodology for achieving the higher knowledge, the knowledge of Brahman, through a process of “knowledge by identity”. This process is now explained. The Upanishad calls for a “one-pointed” concentration on the Eternal, the Absolute, the Immutable so that the ego-individuality is absorbed into the Brahman. This is what Sri Aurobindo calls the “refusal of the ascetic” in The Life Divine. The seeker of Brahman is asked to put aside all focus on the outer world, the lower knowledge or any fulfillment related to the individual in the outer world and fix all the attention and focus on the Brahman.
The Upanishad also introduces the mystic syllable OM as the force that can propel the awareness towards what is elsewhere called a state of Samadhi. OM has been described elsewhere as having enormous significance relating to the 4 states of awareness, waking, dream, sleep and transcendent consciousness. But it is not simply the significance to the mind that makes it important. It is also called the primordial sound that vibrates with the sound of the universal creation. Repetition of this syllable can bring a profound quiet to the “mind stuff” (chitta) described in Patanjali’s Yoga Sutras, and can help guide the seeker to a state of transcendent awareness.
Kirlian photography that captures projection of the human aura has shown tremendous increase of auric energy and coherence of that energy when the subject repeats the word OM. It is clear that this is not intended to be an intellectual understanding of a concept contained in a word or syllable, but is itself a powerful force for realisation when used, as the Upanishad colorfully puts it as “the bow” with the soul as “the arrow” to achieve the realisation of Brahman.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210