1.3.01 and 02

ṛtaṁ pibantau sukṛtasya loke guhāṁ praviṣṭau parame parārdhe। chāyātapau brahmavido vadanti pañcāgnayo ye ca trināciketāḥ ॥1॥ yaḥ setur ījānānāṁ akṣaraṁ brahma tatparam। abhayaṁ titīrṣatāṁ pāraṁ nāciketaṁ śakemasi ॥2॥

The knowers of Brahman, the five-fire sacrificers, and those who have thrice performed the Naciketas rite, having entered the cave in the supreme highest quarter say - "The two, shade and heat", are drunk by the performers of ṛtam, i.e. those engaged in naturally ordained routine, in this benevolent world. The Naciketas are able to reach that which is the bridge for the sacrificers, the fearlessness and the far shore - for those who seek to cross, the imperishable supreme Brahman.

Bhāṣya 1.3.01 and 02

A testimonial is provided where the critical word 'chāyātapau' is explained as the two forms of the lord - for the wise he is like shade ('chāyeva'), for the sinners he is heat ('ātapaḥ').

ātmāntarātmeti vibhuḥ eka eva dvidhā sthitaḥ। sa viṣṇuḥ parame vāyau parebhyo'pyṛddharūpake॥ śubhān pibati bhogān saḥ chāyeva viduṣāṁ prabhuḥ। ātapaḥ pāpināṁ nityaṁ maryādā viṣṇuyājinām॥ saṁsārasya ca pārasthaḥ sa viṣṇurdvisvarūpakaḥ॥

"The all-pervading one is called the self and the inner self, and is established in two forms. He, Viṣṇu, resides in the supreme air, in a form beyond others and full of excellence. He enjoys auspicious pleasures; for the wise he is like shade ('chāyeva'), for the sinners he is heat ('ātapaḥ'), always the boundary for the worshippers of Viṣṇu. He, Viṣṇu of dual form, stands beyond the shore of worldly existence."