Rule sets / OBR / Unoccupied Base Balk
Unoccupied Base Balk
A pitcher may not throw to an unoccupied base unless the throw is part of a play to retire a runner.
If there is a runner, or runners, it is a balk when the pitcher, while touching his plate, throws, or feints a throw to an unoccupied base, except for the purpose of making a play;
When determining whether the pitcher throws or feints a throw to an unoccupied base for the purpose of making a play, the umpire should consider whether a runner on the previous base demonstrates or otherwise creates an impression of his intent to advance to such unoccupied base.
MLB Umpire Development Manual Authoritative interpretation
Official Baseball Rule 6.02(a)(4) provides that the pitcher be charged with a balk if, while in contact with the rubber, he throws to an unoccupied base except for the purpose of making a play.
Play 1: Runners on first and second, pitcher in Set Position. Runner breaks for third base and pitcher throws to third base.
Ruling 1: Legal play.
Play 2: Runners on first and second, pitcher in Set Position. Runner bluffs going to third base and the pitcher throws to third base. However, runner did not go.
Ruling 2: Balk under Official Baseball Rule 6.02(a)(4).
Play 3: With a runner on first base attempting to advance, a right-handed pitcher makes a complete turn and, without hesitating toward first, throws or fakes to second.
Ruling 3: This is not to be interpreted as throwing to an unoccupied base since the runner is trying to advance. See Rule 6.02(a) Comment (B).
The key to understanding the above three plays is for the umpire to use good judgment in deciding whether or not the runner on the previous base demonstrates or otherwise creates an impression of his intent to advance to such unoccupied base. These plays will most likely happen with a 3-2 count and two out.
Another interpretation regarding Official Baseball Rule 6.02(a)(4) concerns appeal plays:
It is NOT a balk for the pitcher, while in contact with the rubber, to throw to an unoccupied base IF it is for the purpose of making an appeal play. (Note that the pitcher does not have to step back off the rubber to make an appeal play.)