वसुस्रंसुध्वंस्वनडुहां दः

द is substituted for the final स् of a word ending in the affix वस् and for the final of 1. स्रंस् 2. ध्वंस् and 3. अनडुह् at the end of a word (in the wider sense निपात एकाजनाङ् 1.1.14, उञः 1.1.17).
The anuvritti of स् is understood here from 8.2.66. It qualifies वस् only, and not the rest. That is, when the word formed by the affix वस् ends with स्, such स् is replaced by द ॥ For a word formed by वस् does not sometimes end in स्, and in those cases the rule will not apply. As स्रंस् and ध्वंस् always end in स्, there is no necessity of qualifying these by the स् of 8.2.66. अनडुह् ends in ह् and so स् cannot qualify it.

Thus विद्वद्भ्याम्, विद्वद्भिः, पपिवद्भ्याम्, पपिवद्भिः with वस् affix. स्रंस् :- उखास्रद्भ्याम्, उखास्रद्भिः ॥ 7.1.70 and 3.2.36. ध्वंस् — पर्णध्वद्भ्याम्, पर्णध्वद्भिः ॥ अनडुह् — अनडुद्भ्याम्, अनडुद्भिः ॥

But when a वस् formed word does not end in स्, the rule does not apply. As विद्वान्, पपिवान् ॥ Here न् is not changed to द् ॥

In the case of वस्, the द is ordained, before any other rule manifests itself; thus in विद्वद्भ्यां this द debars रु; so why should it not debar the rule relating to the elision of the final in विद्वान् also ? This rule sets aside रु, but it does not, however, over-rule the elision-rule, because it is not directly connected with this change.

Why in अनड्वान् the न् is not changed to द्? By the very fact, that नुम् is ordained (विधान-सामर्थ्यात्), this न् will not be changed to द् : otherwise नुम् rule would become superfluous; for it would be easier to say let ह् be changed to द्, and we should get the form अनड्वाद् by this rule without नुम् ॥

Objection:- Is this be so, that the नुम् is not changed to द, because of giving it a scope, then in अनड्वान् अत्र, this नुम् should not be changed into रु by 8.3.9?

Answer:- No; the maxim is that that rule is set aside, with regard to which a particular rule would become useless, if not so over-ruled: but that rule is not set aside which is only an occasion for the application of another rule (यं विधिं प्रति उपदेशोऽनर्थकः सविधिर्बाध्यते, यस्य तु विधेर्निमित्तमेव नासौ बाध्यते) ॥ With regard to द change, the नुम् vidhi is useless (anarthaka), with regard to रु change, it is merely an occasion.

The word पदस्य is understood here also, so the change does not take place in विद्वांसौ and विद्वांसः ॥