Covey had terrorized Douglass for the first six month since his arrival. Covey was skilled at "breaking" difficult slaves, and enjoyed his job tremendously. Covey succeeded in causing Douglass a lot of physical pain during this time, and when Douglass goes to his old master to protest, he is told to go back.
Covey was good at causing slaves to be nervous, and Douglass had been waiting for his retribution in angst.
When Covey finally sets to beat him, Douglass is unwilling to be subservient and he fights back. The battle goes on for over two hours, during which Douglass was able to bloody Covey, but Covey had not bloodied Douglass. In the end, it was Douglass who was the victor.
The moment he stood up for himself, he gained self-confidence and a sense of worth. From this point, he might technically be a slave , but his spirit was no longer enslaved. He declared that if Covey killed him for this later, it would have been worth it.