Al-ṣafwāʾ refers to a rock with a smooth surface. Al-mutanazzal can refer to a descending thing or person; the commentary links it with a figure slipping down a smooth rocky face. Either way, the central image is unimpeded sliding.
There are two corresponding actions:
1. Yazillu al-libdu — the felt pad slides off the horse's back.
2. Kamā zallati al-ṣafwāʾ — a smooth rocky surface provides the comparison for sliding.
The earlier verse used a rock's violent descent to depict force. Here the rock supplies smoothness of surface. A familiar natural object therefore serves two distinct figurative purposes.