Jaiminīya (Talavakāra) Brāhmaṇa 2-8.379

They should make a multiform introductory verse, thinking: 'These juices have been let loose.' For many are they who undergo consecration. But on that point they also say: The introductory verse should just be: 'Purify yourself as the foremost of speech.' For this [verse] is multiform; indeed, all forms are in it. There are two introductory verses, for two sāmans are performed here. It is just as if one were to set out on a great journey after harnessing two [horses] that are the strongest, the most powerful, the best at pulling, and capable of crossing over—such is that. They turn all toward the form of the pṛṣṭha, just as one might turn calves back to their father and mother—such is that. On that point they say: the Agniṣṭoma-sāman should be performed on these [verses]: 'O Agni, harness indeed those who are yours.' For this year is Agni Vaiśvānara. Thus for the harnessing of him: 'Horses, O god, good [workers]'—that is for success; 'The swift ones carry efficiently'—that is for attainment. On that point they say: Those who shift the Yajñāyajñīya [sāman] away from the verses of the Yajñāyajñīya go against sacrifices with a sacrifice. But those establish themselves by sacrifice in sacrifice who shift the Yajñāyajñīya [sāman] onto the verses of the Yajñāyajñīya. Therefore, the Yajñāyajñīya should be performed only on its own [verses].

The Abhīvarta is the Brahman's sāman.

By means of the Abhīvarta, indeed, the gods turned toward these worlds.

Because they turned toward (abhyavartanta), that is the nature of the Abhīvarta (abhīvartatva).

So, that the Abhīvarta is the Brahman's sāman is precisely for turning toward these worlds.