श्वयतेरः

Operational rule vidhi

The अ is substituted for the final of श्वि in the Aorist अङ्.

Word by word (padaccheda)

Target श्वयतेः ṣaṣṭhī · ekavacanaSubstitute / term अः 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

श्वयतेः अः

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)

श्वयतेः → अः

Replace śvayateḥ with aḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The अ is substituted for the final of श्वि in the Aorist अङ्।
As अश्वत्, अश्वताम्, अश्वन् ॥ For the अ of the stem and the अ of the affix, there is the single substitute of the last by 6.2.97. See 3.1.58.

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