Exit velocity is a term that is being used more and more as a measure of hitting. Exit velocity is the speed and direction of the baseball right after contact and it is different than bat speed. Bat speed, as the name implies, is the speed at which the bat is traveling. Bat speed is a factor in producing a high exit velocity, but it is not the only factor and it is not the most important factor.
Bat speed sounds simple enough, but it is not that simple. During a swing, the bat is not moving at a uniform speed. The end of the bat is traveling faster than the knob of the bat(i.e., the end of bat travels further than the knob in the same amount of time). The bat speed that matters is the speed of the barrel as it contacts the baseball. This speed is very difficult to measure.
Using a device that attaches to the knob of the bat for measuring bat speed is not measuring the desired bat speed (i.e., the speed of the barrel at the point of contact), but is measuring hand speed. Too often young hitters using such devices increase bat speed by moving their hands further away from their bodies during the swing. While the hands are traveling a greater distance in the roughly the same amount of time (i.e., are faster), the hitter is casting his or her hands away from the body. Most hitting coaches, however, teach that the hands should stay relatively close to the body for a quick and powerful swing; casting of the hands is counterproductive to this.
Thus, if hitters focus on improving bat speed by measuring hand speed or knob speed, they are mostly likely creating bad habits (i.e., casting the hands), robbing themselves of a quick and powerful swing, and limiting their exit velocities.