Ācārya Abhimānam (taking spiritual refuge in a preceptor/teacher/ācārya) is greatly lauded by Svāmī Pillai Lokāchārya in the Śrīvāchāna Bhūṣaṇam while laying out the nuts and bolts of the process.
Svāmī establishes in Sūtram 461 that it's another one of Viṣṇu's . . .
Upāyam-s (means) that enables all of us Jīvātmā-s to attain Mokṣam, though one can/should also use it as a compliment if they conduct Śaranāgati. That's what Lokāchārya states: "ācārya-abhimānaṇ tāṇ, prapatti pōlē upāyāntaraṅgaḷukku aṅgamumāy svatantramumāy irukkum".
How is Ācārya Abhimānam's efficacy established? Svāmī Lokāchārya in Sūtram 459 states, "'paḷutā-kātoṇṟaṟindēṇ......' eṇgiṟapāṭṭai pūrvōpāyattukku pramāṇamāka anusandhip-patu."
Here, the fourth Thiruvantāḍi, pāsuram 89, is quoted to prove the efficacy of . . .
this upāyam + endorse it. This pāsuram reads: "I have realized an infallible means: worshipping those who resort to the Lord, meditating on the feet of the Lord of the milky ocean, without any distraction — whoever endures this life with humility will destroy accumulated . . .
sins, will open the doors of Vaikuṇṭha and remain there with glory.”
Now, what scriptural sources are considered the authorities of Ācārya Abhimānam? Svāmī Lokāchārya in Sūtram 460 mentions four. First, he mentions pāsuram 10:10 of Āndāl's Nacchiyār Thirumozhi . . .