मनसः संज्ञायाम्

Operational rule vidhi

After मनस् when the compound is a Name, the Instrumental -3rd case endings are not elided before the second member.

Word by word (padaccheda)

Left context मनसः pañcamī · ekavacanaRight context संज्ञायाम् 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.
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").

Full reading

मनसः संज्ञायाम्

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

Commentary

After मनस् when the compound is a Name, the Instrumental -3rd case endings are not elided before the second member.
Thus मनसादत्ता, मनसागुप्ता, मनसासंगता ॥ Why do we say 'when a Name'? Observe मनोदत्ता, मनोगुप्ता ॥

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