Aḥaltu is first-person past and here concerns bringing the whip toward the animal. The fa- in fa-ajdhamat marks the swift consequence of that action: she accelerates. Khabba al-āl adds another type of motion, the shimmering appearance above hot ground.
The verse thus sets actual movement of the camel beside apparent movement in heat haze. This combination makes the desert environment perceptible. The mirage must not be translated as a real stream of water.