A pitcher must begin his throwing motion while standing on the pitching rubber -- which is 60 feet, 6 inches away from home plate. This does not mean pitches are actually thrown from 60 feet, 6 inches away from the plate.
The point at which a pitcher releases the ball is actually a few feet closer to home plate than the pitching rubber itself. Extension quantifies exactly how much closer a pitcher's release point is to home plate. Taller pitchers with long wingspans tend to have the longest Extensions, because their frames allow them to hold on to the ball for a greater distance before releasing it.
Not surprisingly, a longer Extension can be a major advantage to pitchers, because they are essentially shortening the distance between themselves and opposing batters. A pitcher with a longer Extension can make a 93 mph fastball look like a 96 mph fastball. In this regard, Extension is a key component in the Statcast metric "Perceived Velocity."
Watch: Statcast tracks the Extension of Matt Harvey.
In A Call
"His release point was X inches from the front of the rubber"