Bhashya
य एतं मन्त्रराजं नारसिंहमानुष्टुभं नित्यमधीते सोऽग्निं वायुमादित्यं सोममुदकं सर्वान्देवान्ग्रहान्विषं स्तम्भयतीति स्तम्भनफलनिर्देशः ।द्विरभ्यासोऽध्यायसमाप्तिं द्योतयति ॥
In plain words
Fire, wind, sun, moon, water, every god, every planet, poison — daily recitation of this king of mantras brings all of it to a standstill. That is the fruit named here: mastery through stillness. And the doubled line, as ever, marks the chapter's end.
Close to the Sanskrit
'Whoever recites this king of mantras, the Nārasiṃha anuṣṭubh, daily stills fire, wind, the sun, the moon, water, all the gods, the planets, and poison' — this is the specification of the fruit of stilling. The double repetition signals the completion of the chapter.