Tafsir of An-Nazi'at 79:6

The phrase "The Day the Trembler trembles" is in the accusative case due to an implied verbal response. By "the Trembler" (al-rajifah), it is intended the cataclysmic event, or the Blast at which the celestial bodies tremble. This attribution is figurative, as the Blast is the cause of the trembling, or it is a metonymy where the cause of the trembling is itself described as the "Trembler."

It is also permissible to interpret al-rajifah as "the Mover," in which case it is a literal description, as the verb rajafa can mean both "to move" and "to be moved," as stated in the Qamus. This refers to the First Blast.

It has been said that what is meant by it are the stationary bodies whose movement becomes violent at that time, such as the earth and the mountains, due to the Almighty’s saying: "The Day the earth and the mountains tremble." Naming them "the Trembler" is based on the former interpretation, and thus it contains a synecdoche; through this, the benefit of the attribution becomes clear.