This is a term used to describe the position of a pitcher's throwing arm as he picks up the baseball during the cocking phase of a pitch.
In the Inverted V, the pitcher picks up his throwing elbow above his shoulder. The throwing arm resembles an upside-down 'V', so this position is called an inverted 'V'.
This position is basically the same as the Inverted W, but the glove arm is in a position other than an inverted 'V'.
Some examples are below. Visit Chris O'Leary's Death to the Inverted V for more information.
See also: Inverted L.