Created by Tom Tango, Leverage Index measures the importance of a particular event by quantifying the extent to which win probability could change on said event, with 1.0 representing a neutral situation.
For instance, if a team trailing by three runs had the bases loaded with two outs in the eighth inning, the ensuing plate appearance would register an LI above 1.0. This is because the outcome of the game could dramatically change on that one plate appearance. Conversely, if a team trailing by four runs has a man on first with one out in the top of the ninth inning, the ensuing plate appearance would register an LI below 1.0.
Why it's useful
LI can be used to more easily pinpoint the pivotal moments in a particular game and determine how often players face high-leverage situations.