In yazillu al-ghulāmu, yazillu is an imperfect verb and al-ghulām is its subject: the youth slips. The pronoun in ʿan ṣahawātihi refers to the horse, and the phrase identifies the surface from which he slides.
Wa-yulwī bi-athwāb al-ʿanīf contains yulwī, a verb from l-w-y (ل و ي), indicating twisting or pulling out of alignment. The prepositional construction bi-athwāb focuses the action on the rider's garments.
The rhetorical advantage is that force is not merely called “great speed.” Instead, two verbs present two observable consequences of that speed.