दीर्घ इणः किति

Operational rule vidhi

A long vowel is substituted in the reduplicate of the root इ (एति) before a personal ending of the लिट् (Perfect Tense), which has an indicatory क्.

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaTarget इणः ṣaṣṭhī · ekavacanaRight context किति saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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Full reading

इणः किति

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)

इणः → / किति

Replace iṇaḥ with dīrghaḥ when kiti follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

A long vowel is substituted in the reduplicate of the root इ (एति) before a personal ending of the लिट् (Perfect Tense), which has an indicatory क्।
As ईयतुः, ईयुः ॥ These are thus formed इ + अतुस् = य् + अतुः (य् substituted by इ 6.4.81). Then there is reduplication, the य् becomes sthanivat to इ by 1.1.59, and we have इ य् + अतुः and then by the present rule इयतुः ॥ Why do we say before a कित् affix? Observe इयाय, इययिथ ॥

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