ओजस्सहोऽम्भसा वर्तते

Operational rule vidhi

The affix ठक् comes in the sense of 'it exists' after the words 1. ओजस् 2. सहस् and 3. अभस् being in the Instrumental -3rd case in construction.

Word by word (padaccheda)

With / by ओजः- सहः- अम्भसा tṛtīyā · ekavacanaVerb वर्तते tiṅanta

With / by Tṛtīyā (3rd case): accompaniment or means.
Verb A finite verb form (tiṅanta).

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Full reading

ओजः-सहः-अम्भसा वर्तते

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

Commentary

The affix ठक् comes in the sense of 'it exists' after the words 1. ओजस् 2. सहस् and 3. अभस् being in the Instrumental -3rd case in construction.
Thus ओजसा वर्तते = औजसिकः शूरः 'a hero' lit. possessed with energy. So also:- 'a thief.' आम्भसिकः 'a fish.'

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