Reliance on the Ācārya
Ācārya-niṣṭhā is the orientation of one who has surrendered not merely to the abstract Lord but to the Lord through the specific ācārya — and who therefore places unwavering trust in the ācārya's guidance, teaching, and grace. In Sri Vaishnava theology, the ācārya is not merely a teacher but the Lord's own representative (pratinithi) — seeing the Lord in the ācārya is itself a form of surrender to the Lord.
The Ācārya as Prapatti's Vehicle
For many Sri Vaishnavas, the formal act of prapatti is performed through the ācārya: the ācārya places the disciple at the Lord's feet (ācārya-kaṭākṣa — the ācārya's grace-glance), and this act is itself the prapatti. In this understanding, ācārya-niṣṭhā — complete reliance on this relationship — is not separate from prapatti to the Lord but is its very form.
Maturakavi's Model
Maturakavi Āzhvār's extraordinary relationship with Nammāzhvār is the celebrated model of ācārya-niṣṭhā: Maturakavi says explicitly that his goal is not Nārāyaṇa but Nammāzhvār himself — the ācārya is his everything. His Kanninuṇ Sirutāmbu (10 verses) is a celebration of this complete ācārya-niṣṭhā, and Sri Vaishnavas understand it as modeling the ideal relationship of disciple to guru.
Charamopaya-Nirnayam
The text Charamopaya-nirnayam ('Determination of the Ultimate Means') debates whether the ācārya or the Lord is the charamopāya (final means). The conclusion: the Lord is the ultimate means, but the ācārya is the instrument of that grace — ācārya-niṣṭhā and bhagavad-niṣṭhā are not opposed but the former is the experiential form of the latter.