When we let go of our identification with the ego, we become liberated from the little self and exist in the Atman. But the further we become entrenched in an illusory reality of the ego, the greater are our chances of reincarnating from life to life.
The Atman is encased, as it were, in three bodies: the gross (physical), subtle (mental), and causal (ego).
For the causal body -- the innermost sheath of ego-consciousness veiling the Atman -- yoga is the food that nourishes.
Yoga practices transform and renew the mind, spiritualizing one's consciousness so that the Atman may be revealed.
These habits are the building blocks of character, which constitutes the subtle body and -- along with the causal body, or "I"-consciousness -- cloaks the Atman.
(The highest level of ignorance and bondage is thought to occur in the waking state, when the atman identifies itself with the physical being.) With a leap of imagination and extrapolation, this fourth state is thought to have the same essence as Brahman and is the self (jiva) in the individual being.
However, while in both Hindu and Buddhist texts the goddess's severed head symbolizes the destruction of error, the former interpret this error as a reliance on the individual self enmeshed in duality, as opposed to the unified realization of the atman, whereas the latter see all ideas of self - whether the individual or the atman - as deserving of destruction.