Katha-2-3-16-śataṃ caikā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

It has been stated that there is no going; for, the knower who has attained the knowledge that the âtman is the all-pervading Brahman devoid of all attributes and who has untied all the knots due to ignorance, &c., and who has become Brahman even while alive; because, says the sruti ‘ he attains Brahman here’ and also says another sruti ‘ his prânâs do not go out.’ ‘Being Brahman, he attains Brahman; but, for those who attain Brahmalôka by the knowledge of the lower Brahman and by other kinds of worship and for those of an opposite kind who whirl in samsâra this special way is pointed out, with a view to eulogise the fruits of the knowledge of the higher Brahman now treated of; moreover, the knowledge of the agni has been made the subject of a question and a reply and this mantra is begun also for the purpose of stating the process by which the fruit of that knowledge is attained. Here, nerves, one hundred in number, and one other named sushumnâ branch out from the heart of man. Among them, the one named sushumnâ has gone out piercing the head. At the moment of death, one should control the âtman in the heart and make it join that nerve, nâdi; going up by that nerve, one goes through the orb of the sun and attains relative immortality; according to the smriti permanency till the absorption of the elements is spoken of as immortality; or, he attains absolute immortality along with Brahma, in due course of time, having enjoyed incomparable pleasures in the Brahmalôka. At the time of death, the other nerves travel diverse ways, i.e., they become the cause of one, being born again in samsâra alone.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda