"Why is Robertson so difficult to hit? According to Trackman's measurements taken in one American League park last season, Robertson, with his exceptionally long stride and reach, released his fastball seven feet from in front of the pitching rubber -- the largest average extension Trackman measured in that park. The average MLB fastball extension was five feet, 10 inches.
Imagine if Robertson moves the pitching rubber 14 inches closer to home plate every time he pitches. That's the kind of advantage he gains over the average pitcher by releasing his fastball with so much extension. The radar gun (and Trackman) clocks Robertson's fastball at an average of 93 mph. But because Robertson shortens the distance between his release point and home plate, his "effective velocity" is 95 mph. It looks like 93 but gets on a hitter like 95 -- thus the illusion of "hop."
https://www.sbnation.com/2012/5/4/2999268/david-robertson-mariano-rivera-injury-yankees-closer