The knuckle ball is one of the most baffling pitches in baseball because its flight from the pitcher's hands to the plate is unpredictable. It can also be difficult to catch, so catchers often use a slightly larger-than-normal mitt when a knuckle baller is on the mound. The physics behind a knuckle ball's flutter are relatively simple, but mastering the pitch can take a long time.
The Grip
To throw a knuckle ball correctly, grip the ball with the nails of your forefinger and middle finger digging into the ball behind the seams and your thumb and ring finger squeezing the ball underneath. The goal is to take the spin off the ball as you throw it.
Rotation
The perception of a knuckle ball is that it doesn't rotate, but a good knuckle ball actually spins just enough for the seams to change position once as the ball travels to the plate. The seams cause turbulence in the air around the ball. The ball moves in the direction of the leading seam, so if the seams change position as the pitch moves toward the plate, the ball will flutter in different directions in flight.
Subject to the Weather
A little breeze might make the pitch even more unpredictable. In addition to wind, knuckle balls are subject to changes in humidity, temperature and atmospheric pressure.
When They Don't Work
A knuckle ball that rotates too much will come across the plate as a slow fastball, because the seams will be moving too fast and too consistently to result in much change on the throw. You won't get the flutter you need if the ball spins too much. For that reason, the knuckle ball is a rarely thrown pitch. In the Major Leagues, only a handful of knuckle ball pitchers are active at any time.
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