Porter Johnson, a physics professor at the Illinois Institute of Technology and an avid baseball fan, supplies the following answer.
Actually a good knuckleball does have a slight rotation¿it makes between one-half to one complete revolution in traveling from pitcher to homeplate. It is crucial to have the proper orientation of the seams on the ball as the ball travels through the air. Otherwise, the pitch would simply be a "slow fast ball," which would be very easy to hit.
The principal difference between such a "batting practice fastball" and a knuckleball is that the fastball rotates several times in going from pitcher to homeplate. For a knuckleball, the important thing is that the ball rotate about an axis so that the seams are on one side of the front of the ball at one instant, whereas a little later they are on the other side of the front of the ball. The ball will then drift in the direction of the leading seam, and then drift back when the seam becomes exposed on the other side.
The seams produce turbulence in the air flowing around the ball, disturbing the air layer traveling with the ball and thereby producing a force on the ball. As the ball slowly rotates, this force changes, causing the ball to "flutter" and slowly drift. The knuckleball is very difficult to throw well and is sensitive to wind, temperature and, of course, atmospheric pressure.
In my opinion, all the great knuckleball pitchers are retired (Jim Konstanty, Wilbur Wood, Hoyt Wilhelm, Phil Niekro) and no current major league pitcher is a "knuckleballer" in the classic sense, although a few throw it every now and then. Throwing the knuckleball badly will most likely result in a so-called batting practice homerun. But throwing the knuckleball is easier on a pitcher's arm than throwing a curveball or a hard fastball. In fact, a pitcher sometimes learns to throw the knuckleball properly after he has hurt his throwing arm and is trying to regain his former prowess.