शीङः सार्वधातुके गुणः

Operational rule vidhi

For the vowel of शी there is substituted गुण when a सार्वधातुक affix follows.

Word by word (padaccheda)

Target शीङः ṣaṣṭhī · ekavacanaRight context saptamī · ekavacanaSubstitute / term prathamā · 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").
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

शीङः

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)

शीङः → /

Replace śīṅaḥ with guṇaḥ when sārvadhātuke follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the vowel of शी there is substituted गुण when a सार्वधातुक affix follows.
As शेते, शयाते, शेरते, but शिश्ये before the ardhadhatuka affix 3.4.115. Though these sarvadhatuka affixes were ङित् 1.2.4 and would not have caused guna (l. 4.5), they do so by virtue of the present sutra. The word शीङ् is read in the sutra with the anubandha ङ्, in order to indicate that the rule does not apply to यङ् लुक् as शेशीतः, शेश्यति ॥

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