SB 11.14.34
hrdy avicchinam omkaram
ghanta-nadam bisorna-vat
pranenodirya tatratha
punah samvesayet svaram
Translation:
Beginning from the muladhara-cakra, one should move the life air continuously upward like the fibers in the lotus stalk until one reaches the heart, where the sacred syllable om is situated like the sound of a bell. One should thus continue raising the sacred syllable upward the distance of twelve angulas, and there the omkara should be joined together with the fifteen vibrations produced with anusvara.
Purport:
It appears that the yoga system is somewhat technical and difficult to perform. Anusvara refers to a nasal vibration pronounced after the fifteen Sanskrit vowels. The complete explanation of this process is extremely complicated and obviously unsuitable for this age. From this description we can appreciate the sophisticated achievements of those who in former ages practiced mystic meditation. Despite such appreciation, however, we should stick firmly to the simple, foolproof method of meditation prescribed for the present age, the chanting of Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare.