I acknowledge this graphic from the Net.

Shiva Says in his 17th dharana in Vigyana Bhairava Tantra

Focus on the letters first to last

Leading to the void in the pranava

Through that become the void 

Shiva uses the word varna in this dharana to clarify that  one should focus on each of the individual syllables, letters and sounds of AUM to reach the void.  These syllables are also called matras, which are the letters that the word is broken into. In AUM, these are three discernible matras, A, U and M. 

The sound of A, as in Ahhh, comes from the mouth, loud and clear. The next sound U, as ooooh, comes with the lips pursed from the throat region. The last sound M, as in mmmmm, comes with the lips closed from deep with in the chest.

Many explanations are given for this in the Mandukya Upanishad. A stands for vaishvanara, the waking state, being aware of which, one masters the senses and gains what they want. U stands for taijasa, the dream state, being aware of which, one masters dreams and gain wisdom. M stands for prajna, the deep sleep state, being aware of which, one stills the mind, finds oneself and helps everyone grow.

All together AUM transcends its individual letters and stands for the Fourth State turiya, beyond birth and death, everlasting bliss, becoming the SELF.

Practise this dharana by being aware of your inner state as you intone each of the syllables, as you exhale. Watch where the sounds come from. Watch how you feel within. As you move from the loud, through the whisper, to the mental, and finally to silence, be aware of each of the chakras, energy centers, the sound comes from.

A comes from the ajna, the awake consciousness. U comes from the visuddhi, the dream consciousness. M comes from the anahata, the deep sleep causal consciousness. See if the awareness shifts to the sahasrara as you reach the silence.  Don’t force yourself. Let it happen if it does.

Most explanations of this dharana stop at the anahata, the deep sleep causal consciousness of prajna. I find the space after the last syllable M being the most valuable.  Stay with the silence after the M, and experience the void. Some say it’s the bindu, some say it’s beyond the bindu, in the sahasrara. why quibble, just experinece it. 

This dharna leads into the practice of Yoga Nidra, of which i shall talk later.