From BR Bullpen
In baseball, the term knuckle-curve refers to three entirely different pitches.
The first, more common pitch called the knuckle-curve, is really a standard curveball, thrown with one or more of the index or mean fingers bent. According to practictioners, this gives them a better grip on the ball and allows for tighter spin and greater movement. In all other respects, this knuckle-curve is identical to the standard curveball. This version of the knuckle-curve is currently used by Major League pitchers Mike Mussina and Cliff Lee. As it is really not a knuckleball at all, some pitching coaches have called this pitch a spike curve, though this term is rarely used.
The second type of knuckle-curve is a breaking pitch 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 predictable. Because the 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. The most famous practitioners of this type of knuckle curve are Burt 'Happy' Hooton, who pitched for the Chicago Cubs and the Los Angeles Dodgers from the mid 1970s to mid 1980s, and St. Louis Cardinals closer Jason Isringhausen.
The third type of knuckle-curve was thrown by Dave Stenhouse in the 1960s. Stenhouse's knuckle-curve was thrown like a fastball but with a knuckleball grip. Stenhouse discovered that this pitch had excellent movement, and when he came to the majors, he utilized it as a breaking pitch. Furthermore, this pitch may have been the same as the knuckleball thrown by Jesse Haines and Freddie Fitzsimmons.