How does the defense of self-defense work?

The best way to answer this question is to give an example. If someone is about to hit you, and you are afraid that you are about to be hit, and you hit them before they hit you, that is an intentional touching on your part. That is a battery. However, because you were in fear that you were going to be hit, you can raise the defense of self-defense saying, "Yes, I intentionally touched that person, but I did so because I was in fear of being hit."