harer hardam athasina
manim jahara murdhanyam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Suta Gosvami said: Just then Arjuna could understand the motive of the Lord by His equivocal orders, and thus with his sword he severed both hair and jewel from the head of Asvatthama.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Contradictory orders of different persons are impossible to carry out. Therefore a compromise was selected by Arjuna by his sharp intelligence, and he separated the jewel from the head of Asvatthama. This was as good as cutting off his head, and yet his life was saved for all practical purposes. Here Asvatthama is indicated as twice-born. Certainly he was twice-born, but he fell down from his position, and therefore he was properly punished.