تَرْمِيهِمْ بِحِجَارَةٍ مِنْ سِجِّيلٍ
They pelted them with stones of baked clay. (الفيل: 4)
The previous verb arsala is perfect, while tarmīhim is imperfect. Within a past narrative the imperfect can make the action vivid and ongoing before the mind, as if the pelting were unfolding before the listener.
The clause may be related to the preceding birds through a descriptive or circumstantial grammatical analysis. The key pedagogical point is the pictorial force of the tense shift, not an unsupported claim that one parsing is the only imaginable one.
Bi-ḥijāratin min sijjīl clarifies the means and kind of stones without specifying their count or weight.