Idhā can introduce a recurrent or anticipated circumstance. Here idhā hiya naṣṣathu means “when she lifts it,” referring to her neck. Naṣṣat comes from the root n-ṣ-ṣ (ن ص ص) and conveys raising or making prominent; the object suffix -hu points back to the neck.
Hiya explicitly identifies the woman as the agent. Though naṣṣat is a perfect form, with idhā it can present a general or repeatable circumstance rather than only a single completed event.
Rhetorically, the phrase adds movement: instead of a static neck shape, we see the motion of lifting the neck.