The augment num is added after the root-vowel in radh and jabh before affixes beginning with a consonant.,
Thus रन्धयति, रन्धकः, साधुरन्धी, रन्धो वर्तते ॥ जम्भयति, जम्भकः, साधुजम्भी, जम्भंजम्भम्, जम्भो वर्तते ॥ Though the वृद्धि rule is subsequent, it is superseded by the augment. Why do we say 'before an affix beginning with a vowel'? Observe रद्धा, जभ्यम् ॥,