The affix vun comes after the words 1. pāda and 2. śata when preceded by a numeral, the sense being that of distributive relation, and the final of the stem is elided.,
Pratipadikas preceded by a numeral and ending in the words पाद or शत take the affix वुन् when a distributive sense (वीप्सा) is to be expressed: and by the addition of this, the final is elided.
By (V1.4. 148) the final अ of pada and sata would have been elided before the taddhita affix vun; even if such elision was not taught in this rule. The repetition of the word लोप in this rule, therefore, indicates that the elision ordained here is not one caused by the affix. For if the elision had been caused by the affix, then by 1.1.57, such लोपादेश or \substitute\ would be equivalent to the principal (sthanivat); and would thus debar the application of the rule 6.4.130 by which पाद् pad (and not pada) is changed into पद् ॥ In other words, the lopa taught by यस्येति च is परनिमित्तक or caused by some thing which is subsequent, namely by a taddhita affix or by इ or ई affix; therefore, this lopa would be sthani-vat for the purposes of the rule which would apply to something preceding it (1. 1. 57). Therefore, pad becomes sthani-vat to pada. Therefore, the anga or base ending with 'pada' will be called भ, and not the base ending with 'pad', and therefore 6.4.130 which applies to भ bases will not apply, and there will be no substitution of पद् for पाद्, as there is not in the form पादाय. But we want such substitution and hence the employment of words \the final of the stem is elided\.
Thus द्वौ द्वौ पादौ ददाति \he gives two quarters to each\ = द्विपदिकां ददाति (द्विपाद + वुन् = द्विपाद् + वुन् = द्विपत् + वुन्) 6.4.130 = द्विपदिका 7.3.44 by which अ of अक् is changed into इ). Similarly द्वे द्वे शते ददाति = द्विशतिकां ददाति ॥
The compounds above formed are taddhitartha Tatpurusha compounds, viz. compounds containing the sense of a taddhita affix, by rule (II. I. 51). After the compound is formed, then the affix वुन् is added. Thus first we have द्विपाद and द्विशत compounded by (II. I. 51) and then the affix is added. The words पाद and शत when not so compounded, take वुन् affix only in the feminine.
Why do we say \of the words पाद and शत\? Observe द्वौ द्वौ माषौ ददाति, no affixing. Why do we say \preceded by a numeral?\ Observe पादं पादं ददाति ॥ Why do we say \in a distributive sense\? Observe द्वौपादौ ददाति \He gives two quarters:\ द्वे शते ददाति ॥
The enumeration of पाद and शत is useless, as the affix is found after other words also. As, द्विमोदकिकां ददाति \he gives two cakes to each\. त्रिमोदकिकां ददाति ॥,