वेश्च स्वनो भोजने

Operational rule vidhi

The ष् is substituted for the स् of स्वन् after वि and अव when the sense is 'to smack while eating'.

Word by word (padaccheda)

Left context वेः pañcamī · ekavacanaIndeclinable च avyayaTarget स्वनः ṣaṣṭhī · ekavacanaRight context भोजने saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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

वेः च स्वनः भोजने

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

Commentary

The ष् is substituted for the स् of स्वन् after वि and अव when the sense is 'to smack while eating'.
Thus विश्वणति, ष्यष्वणत्, विषष्वाण, अवष्वणति, अवाष्वणत्, अवषष्वाण ॥ That is, he makes sound while eating, he eats with a smack.

Why do we say "when making a smacking sound in eating"? Observe विस्वनति मृदङ्गः "the drum sounds".

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