are you talking about the little black piece on the upper-rear part of the rudder? If that's what you're talking about, it's an adjustable piece. You can adjust it to make your boat track to your liking when you let go of the wheel.
I heard that skiers liked their boats to pull to one side or the other. The reason behind that is so the boat wouldn't wander at all when they were driving through a course. It would make the steering very tight. I also heard that CC used to make their rudders beveled on one side and sharp on the other to help compensate for the driver's weight.