Rule sets / OBR / Throw Not to First or Third Base Balk
Throw Not to First or Third Base Balk
When throwing to first or third base, the pitcher cannot throw to a fielder who is playing off the base and not attempting to retire a runner.
MLB Umpire Development Manual Authoritative interpretation
In addition, the pitcher shall be charged with a balk if, while in contact with the rubber, he attempts a pickoff at first or third base and throws to the fielder who is either in front of or behind the base and obviously not making an attempt at retiring the runner. However, there is no violation if the pitcher throws the ball directly to first or third base in this situation. Also not that there is no violation if the pitcher attempts a pickoff at second base and throws to an infielder who is in front of or behind the base (i.e., this violation is only in reference to pickoffs at first and third base).
There is no violation if a pitcher attempts a pickoff at second base and seeing no fielder covering the bag, throws to the shortstop or second baseman, neither of whom is in the vicinity of the bag nor is making an actual attempt to retire the runner.
Video examples
Left-handed pitcher Jaime García fails to throw to a fielder at first base who is attempting to retire the runner.
Right-handed pitcher Kevin Gausman's pickoff throw goes to the third baseman who is not attempting to retire the runner.
Right-handed pitcher Jake Peavy makes a pickoff attempt to the first baseman who is playing off the bag.
Lindsay breaks down this balk by Oakland right-handed pitcher José Leclerc who make a pickoff attempt to the third baseman who was playing off the bag.