रक्षोयातूनां हननी

Operational rule vidhi

The affix यत् comes in the sense of 'killer' after the words 1. रक्षा and 2. यातू in the Genitive -6th-Case in construction, in the छन्दस् (Vedas).

Word by word (padaccheda)

Target रक्षस्- यातूनाम् ṣaṣṭhī · bahuvacanaSubstitute / term हननी prathamā · ekavacana

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.

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Full reading

रक्षस्-यातूनाम् हननी

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)

रक्षस्- यातूनाम् → हननी

Replace rakṣas-yātūnām with hananī. Inherited words (anuvṛtti) may add further conditions.

Commentary

The affix यत् comes in the sense of 'killer' after the words 1. रक्षा and 2. यातू in the Genitive -6th-Case in construction, in the छन्दस् (Vedas).
That by which anything is killed is called हननी ॥ Thus रक्षस्य॑ and यातव्य॑ meaning 'that which kills the demons called Rakshas and Yatus'. As याते अग्ने रक्षस्या तनः i. e. रक्षसां हननी 'O Agni thy bodies are killers of Rakshas'. So यातव्याः तनूः ॥ The word is in the plural as a mark of respect.

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