“There are two different energies which are in conflict – Ha and Ṭha.
Because of these two energies we are not in our natural state.
Why do they never meet?
Because there is a big obstacle in the form of a coiled serpent.
Further we add to the obstacle rather than reduce it.
Through practices we can use the fire that is in our system to slowly reduce the obstacle.
When this happens the two have the possibility of entering Suṣumnā and become one.
When this happens there is the union of Ha and Ṭha.”
– TKV Desikachar

“The great yogin Yājñavalkhya said that the constant and intensive practice of Prāṇāyāma brought Prāṇa and Agni together, and gradually the obstacle at the base of the Suṣumnā would be totally dissolved.
He gave this block the name ‘Kuṇḍali’ meaning coiled or ‘Kuṇḍalinī’ meaning ‘rolled up’ in other texts.
Kuṇḍalinī represents that which blocks access to the central energetic channel. When this obstacle is eliminated, Prāṇa penetrates and begins to to rise in the central channel.
This is the most precise description we have of the process. This is also the most clear and coherent.”
– ‘Concering the Cakra’ by TKV Desikachar

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Prāṇa – Its origin, function and malfunction