Yusuf Zuck's arm was pierced through by that knife, but fortunately, it didn't hit the bone; it grazed the edge of the bone.
That was because he reacted quickly and dodged. If he hadn't dodged, it's likely that the knife would have stabbed not his arm, but his chest.
One had to admit that the Grandmasters of internal martial arts were monsters; they had reached that level of using Qigong to control force, defying natural science.
Take Tripp Greer, for example, who could jump several meters high and then stand on tree branches without falling. This kind of skill, this defiance of the laws of physics, was something very few people could achieve.
However, no matter how advanced in martial arts one was, they were still vulnerable to kitchen knives. Yusuf Zuck was not a practitioner of internal martial arts, and he hadn't even fully mastered external martial arts, but he still managed to kill the so-called Qigong master with his own skills and his wits in adapting to the situation.