Remember when, as a kid, your flights of fancy led you to pretend you were batting for your favorite team in a situation of the utmost importance? That situation was not the top of the third, with two out and none on, yeah? Instead, we’re talking bottom of the ninth, bases loaded, maybe two out, that sort of thing. That gradient of importance is what Leverage Index is meant to capture.
To get a little more technical, the idea is that for every combination of half-inning, score, baserunners, and outs, there’s a historical win expectancy based on all the baseball games played before.