Ẓalla is a defective verb that here conveys a state or action continuing during the day. Al-ʿadhārā serves as its subject, and the verbal clause beginning yartamīna functions as its predicate.
Yartamīna belongs to Form VIII, related to irtamā–yartamī. Here it describes reciprocal tossing among participants. It is a third-person feminine plural imperfect, marked by nūn al-niswah.
Thus fa-ẓalla al-ʿadhārā yartamīna does not merely report a single instant; it depicts a scene unfolding over time, with interaction among the women.