The horizontal distance, directed toward the catcher, from set point to release of the ball. This is a measure of how far the pitcher reaches toward home when throwing the ball.
Extension is important for pitches because it essentially shortens the distance and puts them closer to the plate. The average extension of a MLB pitcher is 6.5 feet. An extension longer than 6.5 feet will make the pitch appear faster to the batter, giving the batter less time to recognize the pitch. An extension shorter than 6.5 feet will make the pitch appear slower to the batter, giving them more time to recognize the pitch.
Real Life Example:
A 90 mph major league fastball thrown with an extension of 7.5 feet looks as fast as a 92 mph fastball; whereas, a 90 mph fastball with an extension of 5 feet looks like a 88 mph fastball.