Well, since you asked... :roll:
There are 3 velocities that we can be concerned with:
(1) Real Velocity
(2) Perceived Velocity
(3) Effective Velocity
Real velocity is simply the velocity reported by a radar gun.
Perceived velocity takes into account distance and the resulting batter reaction time. For example, an 85mph pitch thrown from distance X is equivalent to an 88mph pitch if that 85mph pitch is thrown from distance X - 12". In other words, moving your release point closer to home plate by 12" is equivalent to throwing 3mph faster.
Effective velocity takes into account pitch location and the batter's reaction time. When a batter gets an outside pitch, he normally lets the pitch come deeper in the contact zone and, therefore, has more time to see and react to the pitch. On the other hand, when a batter gets an inside pitch, the batter needs to get started early to get the fat part of the bat to meet the ball out front. In this case, he has less time to see and react to the pitch. Perry Husband of Hitting Is a Guess (and others?) have determined that the outside pitch is equivalent to a pitch thrown about 6mph slower than a pitch down the middle in terms of the amount of time the batter has to see and react to the pitch. The inside pitch is equivalent to a pitch thrown about 6mph faster than a pitch down the middle. Pitches thrown off the plate are even more extreme while pitches over the plate are closer to the pitch down the middle. Now this is intuitively well-known. Pitchers have long thrown off-speed away and heat in. So, with respect to this specific issue, Husband and others have really just quantified things precisely.
But there is another piece to their research which is more interesting. It involves a concept known as "at risk pitches". An at risk pitch is one whose effective velocity is within 6mph of the previous pitch. They've gone through about 9 years worth of televised games and computed what they call a "hard hit batting average" and have the data to prove that at-risk pitches increase batters' hard-hit averages. So it is important to come up with pitch sequences that avoid at-risk pitches. For example, an 86mph fastball away followed by a 74mph curveball inside is a sequence that produces an at-risk pitch. The 86mph fastball away has an effective velocity of 80mph. The 74mph curveball inside also has an effective velocity of 80mph. The fastball gets the batter's senses "tuned in" to that 80mph speed (and reaction time) and that helps him be on time for the next 80mph pitch. It's interesting research and this, I believe, is what Perry Husband (of "Hitting Is a Guess") covers in his book.