Rule sets / OBR / 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;
Rule 6.02(a)(4) Comment: 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.
TODO: describe the Unoccupied Base Balk in plain English.
TODO: explain the rationale — what the rule is protecting against.
TODO: what the pitcher should do differently.