आतो युक् चिण्कृतोः

Operational rule vidhi

A root-अङ्ग (stem) ending in आ gets the augment युक् (य्) before the Aorist sign चिण् which causes also the वृद्धि of the root-vowel.

Word by word (padaccheda)

Target आतः ṣaṣṭhī · ekavacanaSubstitute / term prathamā · ekavacanaRight context चिण्कृतोः saptamī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

Dotted words are technical terms. Hover or tap one to see its card.

Pink letters are it-markers (anubandhas), tags dropped by 1.3.9. Hover or tap one for the rule that marks it, or try the it-letter finder.

Full reading

आतः युक् चिण्कृतोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

आतः → युक् / चिण्कृतोः

Replace ātaḥ with yuk when ciṇkṛtoḥ follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

A root-अङ्ग (stem) ending in आ gets the augment युक् (य्) before the Aorist sign चिण् which causes also the वृद्धि of the root-vowel.
As अदायि, अधायि with चिण्, दायः and दायकः, धायः, धायकः with ण and ण्वुल् ॥ Why चिण् and krit only? Observe ददौ, दधौ in the Perfect, and चौडिः, बालाकिः with इञ् a Taddhita affix 4.1.96. So also ज्ञा देवता अस्य = ज्ञः ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.