There is the single substitution of the first vowel for the vocalised semi-vowel and the subsequent vowel.,
The word पूर्व is understood here. When a vocalised semi-vowel is followed by a vowel, the vocalised vowel is alone substituted, the subsequent vowel merging in it. Thus यज् + क्त = इअज् + क्त 6.1.15 = इष्टम् (इ + अ = इ); so उप्तम् (वप् + क्त = उअय् + त), ग्रह् + इत = गृअह् + इत = गृहीतं ॥
Had there not been this merging, the vocalisation would become either useless or the two vowels would have been heard separately without sandhi. Thus in वप् + त = उअप् + त, if the अ did not merge in उ, then it would cause sandhi, and उ would be changed to व् by इकोयणचि and the word would again assume the form वप् ॥ But this यणादेश would causes samprasarana rule non-effective, hence it follows that but for the present rule, the two vowels would have been heard separately as उ अप्त ॥
The rule of option in the Chhandas 6.1.106 applies here also. Thus we have यज्यमानौ मित्रावरुणौ or इज्यमानौ ॥ \When you have just said that the two vowels will remain separate and there will be no यणादेश when there is no purvavad-bhava we do you form यज्यमान्; it ought to be इ अज्यमान्\? We have only said that vocalisation rule becomes useless if there was not this rule of merging of the subsequent vowel; but where there is this rule of purva-vad-bhava, and only an option is allowed, then the rule of vocalisation does not become totally useless because it finds its scope in cases like इष्टः &c, therefore when in the alternative it is not applied, there यणादेश will take place naturally, and this ordinary rules of sandhi will apply. The merging, moreover, refers to the vowel which is in the same anga or base with the samprasarana. Thus शकान् ह्वयात = शकह्वा + क्विप् = शकह् उ आ = शखू ॥ Here आ which is in the same अङ्ग with हु merges in to उ ॥ But in forming the dual, we have शकहू + औ ॥ Here औ coming after the samprasarana ऊ does not merge therein, and we have शकह्वौ ॥ Similarly शकहू + अर्थं = शकह्वर्थम् ॥ In fact when once the para-purvatva has taken effect, then subsequent vowels will produce their effect because in the antaranga operation इ अ ष्ट into इष्ट, the purva-rupa is ordained to save the samprasarana from modification, but there is no such necessity when a Bahiranga operation is to be applied.,