Tafsir of Al-Waqi'ah 56:4

(That is, it is convulsed and moved with an intense movement such that whatever is upon it, of buildings and mountains, is demolished.)

Regarding its grammatical status: It is connected to khafidhah (bringing low) or rafiah (raising high) as an instance of al-amal (the noun governing its object), or it is a substitution (badal) for idha waqa'at (when it occurs), as stated by more than one scholar.

Ibn Jinni and Abu al-Fadl al-Razi said: Idha rujjati is in the nominative case (fi mawdi' raf') as the predicate (khabar) for the subject (mubtada') which is idha waqa'at. Neither of them is conditional; rather, they are in the meaning of "time" (waqt), meaning: "The time of its occurrence is the time of the earth’s shaking."

Ibn Malik claimed that idha can be a subject (mubtada') and used this verse as evidence. Abu Hayyan said: It is a substitution for idha waqa'at, and in my view, the answer (jawab) to the conditional clause is explicitly stated, which is His saying: (Then the companions of the right). The meaning is: "If such and such occurs, then the companions of the right—how fortunate they are and how great is their recompense!" That is, their felicity and the greatness of their rank before Allah, the Mighty and Majestic, become manifest at that severe and difficult time for the world. In this [interpretation], there is some remoteness (bu'd).