This is a term used to describe the position of a pitcher's throwing arm as he picks up the ball during the cocking phase of a pitch.
In the Inverted L, the pitcher picks up his throwing elbow leaving his hand and the ball below his elbow. In this position, the elbow does not move above the shoulder. The throwing arm resembles an upside-down 'L', so this position is called an inverted 'L'.
Some examples are below. Visit Chris O'Leary's Death to the Inverted L for more information.
See also: Inverted W.