Here is the actual rule
8.05 If there is a runner, or runners, it is a balk when-
(a) The pitcher, while touching his plate, makes any motion naturally associated with his pitch and fails to make such delivery; If a lefthanded or righthanded pitcher swings his free foot past the back edge of the pitcher's rubber, he is required to pitch to the batter except to throw to second base on a pick off play.
(b) The pitcher, while touching his plate, feints a throw to first base and fails to complete the throw;
(c) The pitcher, while touching his plate, fails to step directly toward a base before throwing to that base; Requires the pitcher, while touching his plate, to step directly toward a base before throwing to that base. If a pitcher turns or spins off of his free foot without actually stepping or if he turns his body and throws before stepping, it is a balk. A pitcher is to step directly toward a base before throwing to that base but does not require him to throw (except to first base only) because he steps. It is possible, with runners on first and third, for the pitcher to step toward third and not throw, merely to bluff the runner back to third; then seeing the runner on first start for second, turn and step toward and throw to first base. This is legal. However, if, with runners on first and third, the pitcher, while in contact with the rubber, steps toward third and then immediately and in practically the same motion "wheels" and throws to first base, it is obviously an attempt to deceive the runner at first base, and in such a move it is practically impossible to step directly toward first base before the throw to first base, and such a move shall be called a balk. Of course, if the pitcher steps off the rubber and then makes such a move, it is not a balk.