Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 1, Verse 29
Pada 1, Sutra 29
Tatah pratyak-cetenadhigamo ‘pyantarayabhavas ca
From that comes the turning inwards of consciousness and the disappearance of obstacles.
With the conscious chanting repetition of AUM the mind turns inward towards its own inner Self awareness and with it comes the gradual disappearance of the obstacles to that inner awareness (which will be listed in the next sutra or verse).
Patanjali has already defined in Sutra 2 what Yoga is: Yoga is the cessation of the fluctuation of the mind-stuff. The nature of the mind with its usual distractions through the senses and getting caught up in the incessant cogitation, perturbation and agitation of its own thought processes is usually focused outwards or in layering (mentally chewing on its own non-sense), either disconnecting from or veiling the Inner Self, that Eternal Witness Consciousness.
Attachment to the mind-stuff reinforces and solidifies identity with the temporal and impermanency which in turns feeds the insecurity and false sense of self of the ego.
Chanting the Pranava (AUM) with full awareness both on its vibration and its meaning will turn the mind inwards gradually dissolving any obstacles imposed or surfacing from the mind itself. With continued practice (i.e. concerted Sadhana) the ego slowly transforms from its preoccupation with ‘I, my and mine’ to ‘we, us and ours’ and finally to ‘There is Only the One’.
One of the greatest imagery and Cosmic Principle known to humans which is most efficacious in arresting the mind getting caught up on the wheel of Samsara and come to the realization of its own perennial essence is Kali MA wielding the severed head. Kali MA is none other than Brahman, the One Without a Second, the Ineffable One in action. On that basis the greatest of Mantra could very well be: OM MA!!!