स्फायो वः

Operational rule vidhi

For the final of the stem स्फाय् is substituted व् in the Causative.

Word by word (padaccheda)

Target स्फायः ṣaṣṭhī · ekavacanaSubstitute / term वः 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.

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

स्फायः वः

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)

स्फायः → वः

Replace sphāyaḥ with vaḥ. Inherited words (anuvṛtti) may add further conditions.

For the final of the stem स्फाय् is substituted व् in the Causative.
As स्फावयति ॥

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