This one is interesting....
The second type of knuckle curve is a breaking ball that is thrown with a grip similar to the knuckleball. Unlike a knuckleball, which spins very little, a knuckle curve spins like a normal curveball because the pitcher's index and middle fingers push the top of the ball into a downward curve at the moment of release. Since only two fingers produce the spin, however, a knuckle curve does not spin as fast as a curveball, meaning the break is less sharp, and less predictable. Because this knuckle curve can be thrown with the same general motion as a fastball, it is more deceptive than a normal curveball. This kind of knuckle curve is rare—it is easier to control than a standard knuckleball, but still difficult to master.