Honestly, when I originally saw spin rate in the title of the thread I was thinking more about tennis than baseball. Tennis is all about spin 24x 7 x 365, every shot you have to evaluate what your opponent has done to the ball. Some of you that know me well know that I play a lot of tennis. In this thread, we've discuss spin rate being about measuring movement for pitchers. There is something else to it...impact When you are hitting a baseball or a tennis ball that has a lot of spin or movement it is different from player to player. I've played against some really heavy topspin players (think Rafael Nadal) who hit the ball so different from anybody on the planet that they can almost knock the racket out of your hand because of their unique heavy spin. Topspin allows tennis players to hit the ball harder and keep it in the court. The mens and womens tennis tours have been measuring mph, topspin, underspin, net margin, depth, etc and other metrics for a long, long time. Baseball is catching up, quickly. Just like Nadal, I've seen MLB pitchers do the same with MLB hitters. There is one guy (Blake Treinen...Dodgers just signed him) whom I watched many times in person when he was with the Nationals. He is Mr. Spin Rate. This guy was busting hitters bats left and right like he was getting a Christmas bonus for breaking bats. So, where I'm going with this is that it has to do with movement but it also has to do with impact and the opposing players ability to control the ball at impact whether it is tennis or baseball. The heavier the spin rate the tougher it is to square up a ball, but also control where you want it to go.