तिरसोऽन्यतरस्याम्

Operational rule vidhi

The विसर्ग of the word तिरस् is optionally converted to सकार in presence of कवर्ग / पवर्ग letter.

Word by word (padaccheda)

Target तिरसः ṣaṣṭhī · ekavacanaRight context अन्यतरस्याम् 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").

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Full reading

तिरसः अन्यतरस्याम्

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

Commentary

The विसर्ग of गति तिरस् is optionally changed to स् before a hard guttural or a labial.
Thus तिरस्कर्त्ता, तिरस्कर्त्तुम्, तिरस्कर्त्तव्यम्, or तिरः कर्त्ता, तिरः कर्त्तुम्, तिरः कर्त्तव्यम् ॥ The word गतेः is understood here also. Therefore no option is allowed here, तिरः कृत्वा काण्डं गतः, where तिरस् does not mean 'disappearance'. 1.4.72.

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