The u is the substitute of ru (the r substitute of a final s [[8.2.66]]) when it is followed by a soft consonant and preceded by an apluta short a,
The हश् pratyahara includes all sonants or soft consonants. Thus पुरुषो याति or हसति or ददाति &c.,
हशि च
Adhyāya 6 · Pāda 1 · Rule 114