निजां त्रयाणां गुणः श्लौ

Operational rule vidhi

गुण is substituted for the vowel of the reduplicate in the Reduplicated Present form (श्लु) of निज् , विज् and विष्.

Word by word (padaccheda)

Target निजाम् ṣaṣṭhī · bahuvacanaTarget त्रयाणाम् ṣaṣṭhī · bahuvacanaSubstitute / term prathamā · ekavacanaRight context श्लौ saptamī · 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.
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

निजाम् त्रयाणाम् श्लौ

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)

निजाम् त्रयाणाम् → / श्लौ

Replace nijām trayāṇām with guṇaḥ when ślau follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

गुण is substituted for the vowel of the reduplicate in the Reduplicated Present form (श्लु) of निज् , विज् and विष्।
Thus नेनेक्ति, वेवेक्ति, वेवेष्टि ॥ The word त्रयाणां could have been spared in this sutra, as these three roots stand at the end of a subdivision, and निजां would have denoted these three without the word trayanam. The word is however used here for the sake of the subsequent sutra. Why do we say in the reduplicated Present form? Observe निनेज in the Perfect.

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