What could happen in an instant? In dangerous circumstances, it could determine whether a bystander was a hero or a coward; in a duel between masters, it could change the destiny of a sure winner to that of a sure loser and determine one's life and death.
Lü Mian was a typical avenger. He had been secretly practicing very hard, and he didn't enter the martial arts world until he had built up enough confidence and was sure that his kung fu skill was better than his foe's.
He believed in his swordsmanship.
Stabbing the giant roc with a throwing knife and fighting 500 cavalrymen alone both further solidified his confidence.