Tafsir of At-Tur 52:9

(On the day the heaven fluctuates with a [violent] fluctuation)

This is in the accusative case, serving as an adverb of time. Its governing agent is "[a punishment] impending" (waqi') or "[an averter] warding off" (dafi'). Regarding the negation and the implication that its warding off is not negated other than at that time—based on the consideration of the concept (mafhum)—there is no harm in this, as it does not contradict reality; for the Exalted has granted them respite in this world, and [yet] He has not granted them respite. Makki forbade that waqi' act upon it, but he did not mention any evidence for this prohibition, nor is there any evidence for him, as it appears.

The meaning of tamuru (fluctuates) is to be agitated, as Ibn Abbas said, meaning it sways while in its place. In another narration from him, it means it splits. Mujahid said: It rotates. The root of al-mawr is back-and-forth movement in coming and going. It is also said to mean moving in waves, and some say it means rapid flowing. It is applied to flowing in general; they recited [the verse] of al-A'sha: "As if her gait from her neighbor’s house / is the movement (mawr) of a cloud, neither slow nor hasty."