अथास्या ऊरू विहापयति विजिहीथां द्यावापृथिवी इति ।तस्यामर्थं निष्ठाय मुखेन मुखं संधाय त्रिरेनामनुलोमामनुमार्ष्टि विष्णुर्योनिं कल्पयतु त्वष्टा रूपाणि पिंशतु ।आ सिञ्चतु प्रजापतिर्धाता गर्भं दधातु ते ।गर्भं धेहि सिनीवालि गर्भं धेहि पृथुष्टुके ।गर्भं ते अश्विनौ देवावाधत्तां पुष्करस्रजौ ॥६,४.२१ ॥
In plain words
Then he parts her thighs, saying, "Part yourselves, heaven and earth." Having united with her and joined mouth to mouth, he strokes her three times downward, saying: "Let Viṣṇu prepare the womb; let Tvaṣṭṛ shape the forms; let Prajāpati pour in; let Dhātṛ lay the germ in thee. Lay the germ, Sinīvālī; lay the germ, thou of the broad braid. May the two Aśvins, the gods garlanded with lotus, lay the germ in thee."
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
अथास्या ऊरू विहापयति 'विजिहीथां द्यावापृथिवी' इत्यनेन । तस्यामर्थमित्यादि पूर्ववत् । त्रिः एनां शिरःप्रभृति अनुलोमामनुमार्ष्टि 'विष्णुर्योनिं' इत्यादि प्रतिमन्त्रम् ॥
In plain words
'He parts her thighs' — that is done with the mantra 'let heaven and earth open.' 'He settles himself in her' and the rest follow exactly as already explained. He strokes her three times, starting at the head and moving with the hair, saying 'may Viṣṇu shape the womb' and the rest of that formula, one line for each stroke.
Close to the Sanskrit
'Then he causes her two thighs to part' — by this [mantra] 'let heaven and earth separate.' 'Having settled the organ in her' and so on [is understood] as before. He strokes her thrice, starting from the head, along the grain of the hair, [with] 'may Viṣṇu prepare the womb' and so on, mantra by mantra.