Kumayt is a recognized descriptor for a horse's coat, more specific than plain “red.” Libd is felt or a pad used with riding gear. Matn denotes the back, and ḥāl matnihi identifies the pertinent contour or position on the back.
Yazillu is an imperfect verb from z-l-l (ز ل ل), “to slip”; zallat is the feminine past form of the same root, paired with the feminine noun al-ṣafwāʾ. Repeating the same action gives the two compared scenes a verbal connection.
The shared quality between back and rock is surface smoothness, not physical composition. That distinction is crucial for explaining the literary figure accurately.