The affix ṭhañ comes in the sense of 'staying' and 'a commentary' after a polysyllabic word having udātta on the final (the word being the name of a thing to be explained).,
This debars अण् ॥ Thus षात्वणत्विकम् 'a commentary of the rules of changes of स and न into ष and ण' ॥ नातानतिकम् 'a commentary on (अनत) udatta and anudatta (नत)' ॥ The radicals षत्वणत्व and नतानत are acutely accented on the final by the rule of a compound having accent on the final 6.1.169
Why do we say बह्वचः 'polysyllabic'? Observe सौपम्, तैङम् &c. A word of two syllables will take ठक् 4.3.72. So the counter example must be of words of one syllable.
Why do we say 'having acute accent on the final'? Observe सांहितम् from संहिता ॥ This word has acute accent on the initial by the rule of Gati-accent 6.2.49,