Avatarika
स यत्रायमात्मा ।संसारोपवर्णनं प्रस्तुतम् ; ‘तत्रायं पुरुष एभ्योऽङ्गेभ्यः संप्रमुच्य’ इत्युक्तम् ।तत्संप्रमोक्षणं कस्मिन्काले कथं वेति सविस्तरं संसरणं वर्णयितव्यमित्यारभ्यते—
In plain words
'That self, at the moment when...' — we are now in the middle of describing how the self moves on. It was already said that this person releases himself from these limbs; what is left is exactly when that release happens, and how — and that is what the text now sets out to describe, in full.
Close to the Sanskrit
'That, when this self...' — the description of transmigration is now under discussion; it was said earlier, 'there, this person, having released himself from these limbs...'; as to at what time, and how, that release occurs, the text now begins, in order to describe the transmigration at length.