From BR Bullpen

A forkball is a pitch that can be considered a variety of change-up. The pitcher grips a forkball by spreading his index and middle finger very wide apart and jamming the ball into the space between them. The forkball should not be confused with the splitter, another pitch thrown with the fingers spread wide apart.

According to Thorn and Holway's 1987 book The Pitcher, Mike Lynch taught himself to throw the forkball but couldn't control it. A few years later he taught the young Bert Hall, who may have been the first pitcher to throw it in the majors, in 1911. The pitch is hard to master and largely disappeared by the end of the 1970s after the splitter gained popularity.

Well-known practitioners of the pitch include Roy Face and Lindy McDaniel, who largely relied on it during their big league career as top relievers. In the 1980s, Dave Stewart and Jack Morris had success with the pitch as part of a more varied arsenal. Others who have had success with it in recent years include Melido Perez, Mel Rojas and Hideo Nomo (the pitch is much more common in Japan than in North America).