No sooner had the mastiff been led into the hall than it sprang at Zhao Dun, the only one wearing a purple robe. Zhao dodged and ran for his life. He kept running round and round in the hall with the dog in hot pursuit. Officals at the scene stood aghast, not knowing wht to do. Ti Miming, the imperial military chief, Ti struck it down with one blow from his melon-shaped weapon. Then, grasping the dog by its jaws, he tore it into two pieces.
依照上段的句子可知事情是已過去的,請問上段中Then, grasping the dog by its jaws, he tore it into two pieces. 為何用現在進行式grasping?