إِذْ تَسْتَبِيكَ
When she captivates you.
Idh locates a scene in time. Tastabīka is an imperfect verb with an implied feminine subject (“she”); the final -ka is the second-person object suffix.
The poet speaks of the beloved and then introduces “you,” drawing the listener into the experience. The preposition in bi-dhī ghurūbin indicates the means by which the attraction takes place: the beauty of the teeth.