अथ यत्रैतदस्माच्छरीरादुत्क्रामत्यथैतैरेव रश्मिभिरूर्ध्वमाक्रमते स ओमिति वा होद्वा मीयते स यावत्क्षिप्येन्मनस्तावदादित्यं गच्छत्येतद्वै खलु लोकद्वारं विदुषां प्रपदनं निरोधोऽविदुषाम् ॥ ८.६.५ ॥
In plain words
But when he goes out from this body, he goes upward by these very rays. He departs with Om, and no sooner has he sent forth his mind than he reaches the sun. This is the door of the world, the entering in for those who know and the barring out for those who do not know.
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
तत्र एवं सति, अथ यत्र यस्मिन्काले अबलिमानम् गच्छतीत्यर्थः, न तु तावतैव कालेनेति विवक्षितम् । किमर्थमा- दित्यं गच्छतीति, उच्यते- एतद्वै खलु प्रसिद्धं ब्रह्मलोकस्य द्वारं य आदित्यः ; तेन द्वारभूतेन ब्रह्मलोकं गच्छति विद्वान् । अतः विदुषां प्रपदनम्, प्रपद्यते ब्रह्मलोक मनेन द्वारेणेति प्रपदनम् । निरोधनं निरोधः अस्मादादित्यादविदुषां भवतीति निरोधः, सौरेण तेजसा देहे एव निरुद्धाः सन्तः मूर्धन्यया नाड्या नोत्क्रमन्त एवेत्यर्थः, 'विष्वङ्ङन्या' इति श्लोकान् ॥
In plain words
In this context, when the text speaks of a person reaching weakness, it refers to the condition itself, not restricted to a specific duration of time. Why does one go toward the sun? It is well known that the sun is the gateway to the world of Brahma; through this gateway the wise person attains that world. Hence 'approach' means that the knower reaches the world of Brahma through this very door. 'Obstruction' means that for the ignorant, passage is blocked right here by the sun; restrained by the solar brilliance within the physical body itself, they fail to ascend through the cranial channel. Thus concludes the verse beginning with 'differently directed'.
Close to the Sanskrit
There, this being so, now where at which time he goes to weakness, this is the meaning, not that it is intended by that time alone. For what purpose does he go to the sun? It is said: that indeed is well known as the door of the world of Brahma which is the sun; by that door-form the knower goes to the world of Brahma. Hence the approach of the knowers, it is approach because one approaches the world of Brahma by this door. The obstruction, obstruction happens to the non-knowers from this sun, that is obstruction, being obstructed right in the body by the solar splendor, they do not ascend by the cranial channel indeed, thus, 'differently directed other', thus the verses.