The root-vowel of ghas and bhas is elided in the chandas (Vedas) before any affix whether beginning with a vowel or a consonant which has an indicatory k or ṅ ,
Thus सग्धिश्चमे सपीतिश्च मे, बब्धांते हरी धानाः ॥ सग्धिः is thus derived : क्तिन् is added to the root अद्; then by 2.4.39, घस् is substituted for अद्; thus घस् + ति = घस + ति (अ being elided by the present sutra) = घ् + ति (स् being elided by 8.2.26), then त is changed to ध, and घ to ग and we have ग्धिः ॥ Then समाना ग्धिः = सग्धिः (समान changed to स 6.3.84). The word बब्धाम् is the Imperative of भस्, thus भस् + श्लु + ताम् = भ भस् + ताम् स् = ब भ्स् + ताम् 6.4.100 = बभ् + ताम् 8.2.26 = ब ब् + धाम् बब्धाम् ॥ This rule of elision being a nitya and a subsequent rule ought to have operated first, but, as a Vedic anomaly, the reduplication takes place first. See 7.2.67
Why do we read \before an affix beginning also with a consonant\? Because the elision takes place before a vowel affix also, as बप्सति = भस् + श्लु + झि = ब भस् + अति (VII. I. 4) बप्सति 8.4.55.
Why having an indicatory क् or ङ्? Observe अंशून् बभस्ति ॥ The च has been added into the aphorism by the Vartikakara.,