प्रवाहणस्य ढे

Operational rule vidhi

Before the affix ढ (एय), the वृद्धि is substituted for the first vowel of the second member of प्रवाहण but optionally so for the first member (i.e. प्र).

Word by word (padaccheda)

Target प्रवाहणस्य ṣaṣṭhī · ekavacanaRight context ढे saptamī · ekavacana

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").

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

प्रवाहणस्य ढे

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

Commentary

Before the affix ढ (एय), the वृद्धि is substituted for the first vowel of the second member of प्रवाहण but optionally so for the first member (i.e. प्र).
Thus प्रवाहणस्यापत्यं = प्रावाहणेयः or प्रवाहणेयः ॥ The affix ढक् 4.1.123, is added, similarly प्रवाहणेयी भार्याऽस्य = प्रवाहणेयीभार्यः 6.3.39, masculation prohibited)). Or we may translate the sutra as "Before the affix ढ, the vowel of प्र in प्रवाहण optionally gets Vriddhi", and omit the rest. The masculation will still be prohibited by 6.3.41, (जाते)

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