Bhashya
अथ आवृत्तेषु अवाङ्मुखेषु पञ्चविधमुच्यते सामोपासनम् ।गत्यागतिविशिष्टा हि लोका ।यथा ते, तथाह-ष्ट्यैव सामोपासन विधीयते यत , अत आवृत्तेषु लोकेषु ।द्यौर्हिंकार , प्राथम्यात् ।आदित्य प्रस्ताव , उदिते ह्यादि त्ये प्रस्तूयन्ते कर्माणि प्राणिनाम् ।अ-न्तरिक्षमुद्गीथ पूर्ववत् ।अग्नि प्रतिहार , प्राणिभि प्रतिहरणादग्ने ।पृथिवी निध- नम् , तत आगतानामिह निधनात् ॥
In plain words
Now, we consider the five-fold meditation on the Sāman chant in relation to the worlds in their reverse or downward-facing order. For these worlds are defined by the cycles of ascent and descent. Just as they exist in this dual movement, so too is the meditation on the Sāman prescribed in accordance with that very perspective. Therefore, in these returning worlds: Heaven is the introductory Hiṃkāra, because it stands first. The Sun is the Prastāva, for it is only when the sun rises that the daily activities of all living beings are set in motion. The intermediate atmosphere is the Udgītha, for the same reason as before. Fire is the Pratihāra, since fire is gathered and brought home by living beings. And the Earth is the Nidhana, the final resting place, because those who descend from the higher realms find their ultimate support and rest here.
Close to the Sanskrit
Now, in the returned, downward-facing [worlds], the five-fold Sāman meditation is spoken of. For the worlds are characterized by going and returning. Just as they [are], so indeed by that view only the Sāman meditation is prescribed, because, therefore, in the returned worlds: Heaven is the Hiṃkāra, due to priority. The Sun is the Prastāva, because when the sun has risen, the actions of living beings are commenced. The atmosphere is the Udgītha, as before. Fire is the Pratihāra, because of fire being brought back by living beings. Earth is the Nidhana, because of those who have come from there finding their rest here.