@kypoou : while MC can't attack, he can redirect the opponent attack to inflict damage against the one who attacked him. (but the MC suffer some injuries in the process).
It's a one trick thing though, because he you don't attack the MC he can't do harm to you.
So he baited all the sect to attack him and redirected the attack power of the strong guy against everyone who attacked him.
@Wolvenworks you're correct, he loses his own life if he attacks. They said that maybe 80 chapters back when Liuli (blue hair girl) was stabbed in the back and Wang Li punched the stabber three times, losing 30 years each time because of his oath. That's how his master, Wang Wu fights too, she baits the opponent into attacking their shield wall and reflects it back