It is called a "balk."
Most basically, a pitcher is restricted to a certain set of motions and one of two basic pitching positions before and during a pitch; if these are violated, a balk is called.
With a runner on base and the pitcher on or astride (with one leg on each side of) the rubber, it is a balk when the pitcher:
It should be noted that the pitcher's acts of spitting on the ball, defacing or altering the ball, rubbing the ball on the clothing or body, or applying a foreign substance to the ball, are not balks. If done intentionally, these actions result in immediate ejection for the pitcher, the result of a rule change made prior to the 2008 season.
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A team has to predetermine and release it's roster before game day. A pitcher that is not assigned to that particular roster cannot come in the game. Another situation that can make a pitcher ineligible is if he were to come out of the game at any point; once a pitcher is replaced (on the mound, at the plate, or on the bases) he cannot return for the remainder of that game.
There are several ways a pitcher can make an "illegal" pitch:
- step towards a base while throwing to the plate.
- not follow the proper routine in either a "wind up" or a "set" position.
- step on the pitching rubber and then immediately throw.
- apply any substance to the ball, including saliva or sweat.
- throw a ball that's been scratched.
- intentionally throw the ball at a batter, particularly after a warning.
Summing up: illegal pitches include balks, spitballs, and beanballs.
One way is if you use vaseline on the ball or rosin on the ball.Another is by throwing a spitball.