فَأَثَرْنَ بِهِ نَقْعًا
Then they raised dust thereby. (100:4)
فَوَسَطْنَ بِهِ جَمْعًا
Then they reached into the midst of an assembled group. (100:5)
The fāʾ of fa-atharna and fa-wasaṭna ties the actions together. Raised dust followed by arrival among the gathered ranks gives the impression of swift progress and completion. Exegetes discuss how bihi refers back within the scene, so a single referent need not be imposed without considering the context.
This fāʾ should be distinguished from fa-man in the closing rules of az-Zalzalah: here the verses sequence a scene, whereas there a general moral conclusion follows the previous disclosure.