text: According to the Official Baseball Rules: "Umpires should bear in mind that the purpose of the balk rule is to prevent the pitcher from deliberately deceiving the base runner."
  Balk actions
  A pitcher is restricted to a certain set of motions and one of two basic pitching positions before and during a pitch. If these regulations are violated with one or more runners on base, an umpire may call a balk. The batter at home plate does not advance on a balk.
  With a runner on base and the pitcher on or astride (with one leg on each side of) the rubber, under Official Baseball Rules, it is a balk when the pitcher:
  Switches pitching position from the windup to the set (or vice versa) without properly disengaging the rubber
  While on the rubber, makes a motion associated with a pitch and does not complete the delivery
  When pitching from the set position, fails to make a complete stop with his hands together before beginning to pitch
  Throws from the mound to a base without stepping toward (gaining distance in the direction of that base)
  Throws or feints a throw from the rubber to an unoccupied base, unless a play is imminent
  Steps or feints from the rubber to first or third base without completing the throw