The affix u comes after the roots dhinva 'to please' and kṛṇiva 'to hurt', when a sārvadhātuka affix denoting the agent follows and in these verbs a is the final substitute of the final i,
The root धिन्व or धिन्वि belongs to the Bhuadi class. Thus धिन्व + उ + ति = धिन् + अ + उ + ति = धिन् + उ + ति 6.4.48 = धिनोति 'he pleases'. It might be asked, why there is no guna of the इ of धिन् by rule 7.3.86 which requires the base to be gunated before a sarvadhatuka or an ardhadhatuka affix. The answer is to be found in sutra 1.1.57; the substitute of अ which is a blank, is like the former occupant (sthanivat), and thus prevents guna.,