Effective Velocity (EV) is the study of pitch speed and how location changes the reaction time by forcing the hitter to hit the ball at a contact point that is different than they were ready for.
When a pitch at 90 MPH is inside or outside, the speed “Effectively” changes because the hitter has to hit the ball earlier or later as though the pitch gained or lost speed. So, in essence, the pitch location has caused the hitter to gain or lose reaction time.
For every 6″ inches that the ball changes laterally, there is a little less than 3MPH gained or lost. EV is like time management – only for reaction time. Bottom line in an at bat, whoever understands how timing works will win most of the battles.