hrt-padma-stham param mama
anvim jiva-kalam dhyayen
The worshiper should meditate upon My subtle form — which is situated within the worshiper’s own body, now purified by air and fire — as the source of all living entities. This form of the Lord is experienced by self-realized sages in the last part of the vibration of the sacred syllable om.
According to Srila Sridhara Svami, the pranava, or omkara, has five parts: A, U, M, the nasal bindu and the reverberation (nada). Liberated souls meditate upon the Lord at the end of that reverberation.