Now... I'm not a pro baseball player, or a pro athlete of any sort but I am still qualified to explain the intricacies of a knuckling baseball. For the layman, a knuckleball is a pitch that flutters around and breaks somewhat randomly. They are usually not thrown hard and are not common as they are very difficult to throw for strikes. The idea is to throw the baseball and put NO spin on the ball. Try it, it isn't easy to do.
In baseball, the usual method to achieve a good knuckle is to dig your nails into the ball, leave a gap between the ball and your palm and PUSH the ball towards home. I can barely throw a ball from a pro mound to the catcher with my noodle of an arm, so just imagine trying to do that without snapping your wrist.
I may not be able to throw a baseball and make it knuckle, but... I don't play baseball particularly well. I AM a pretty decent volleyball player. I can serve with top spin, side spin, deaden the ball or make it heavy. I can also serve a volleyball with NO SPIN and make it knuckle. The how of this serve is less important than the why. What purpose does it serve?
For that, you have to have had a knuckleball thrown at you. In volleyball, where the ball is larger and more forgiving, a knuckling serve will likely force the passer to hit the ball in such a way as to make the setter scramble. If that happens, his team will likely not get a clean spike off the serve. In baseball you need to time and aim your swing on a much smaller, harder and faster ball. In that last second, if the ball dips, you could ground out to the Short Stop, if it stays up, you could pop out to the catcher or if it does something REALLY odd, you could swing and miss all together.
Why doesn't EVERYONE throw them then? Well... here's the thing. First, they're wicked hard to throw. Second, they're mighty hard to disguise, so batters can see them coming. Third, mistakes are pretty dreadful. Ever seen a hanging knuckler? Ever gone to a batting cage and set it to the lowest setting? It's sorta the same thing. Do something wrong and the chance that someone is going to get a pretty good rip... is pretty good.
What makes R.A. Dickey and Tim Wakfield special? Is it the facial hair? Is it the everyman mentality? Is it simply that they can get a knuckling baseball to travel from their hand to a catchers mitt 60 feet away? Yeah... it's probably that one.