In naṣaʾtuhā, the past verb takes -tu as its first-person subject and -hā as its object, referring to the she-camel. The action is urging or driving her forward.
The word turns the strong mount from a static description into an animal actively directed by the poet over a particular road. ʿAlā lāḥib identifies the surface of travel; ka-annahu then presents a comparison for the appearance of that road.