Bhashya
यावत् तदुपभोगनिमित्तस्य कर्मणः क्षयः, संपतन्ति येनेति संपातः कर्मणः क्षयः यावत्संपातं यावत्कर्मणः क्षय इत्यर्थः, तावत् तस्मिंश्चन्द्रमण्डले उषित्वा अथ अनन्तरम् एतमेव वक्ष्यमाणमध्वानं मार्गं पुनर्निवर्तन्ते ।पुनर्निवर्तन्त इति प्रयोगात्पूर्वमप्यसकृच्चन्द्रमण्डलं गता निवृत्ताश्च आसन्निति गम्यते ।तस्मादिह लोके इष्टादिकर्मोपचित्य चन्द्रं गच्छन्ति ; तत्क्षये च आवर्तन्ते ; क्षणमात्रमपि तत्र स्थातुं न लभ्यते,
In plain words
For as long as the karma that fuels their experience remains, souls remain in the lunar sphere; when that merit is spent, they descend. The phrase 'they return again' implies that they have made this journey many times before. Accumulating merit through ritual and charity here on earth, they ascend to the moon; and when that merit wears away, back they come, unable to linger there for even a single moment.
Close to the Sanskrit
As long as there is the exhaustion of that action which is the cause of enjoyment, they fall down by which path, that is the descent and the exhaustion of action, meaning as long as the exhaustion of action. Having dwelt there in that lunar sphere, then immediately afterwards they return precisely by this described path. From the usage 'they return again', it is understood that even before this they had gone to the lunar sphere and returned repeatedly. Therefore, having accumulated sacrifices and other actions here in this world, they go to the moon; and upon the exhaustion of that, they return; not even a moment of staying is obtained there.