Assuming we are trying to move a weight as fast as possible regardless of how heavy the weight is, what we soon realize is that the heavier the load is, the slower the bar moves. Now, research has shown us that in dynamic exercises, like squats, bench and deadlifts (tons of other exercises too) the relationship between load and velocity is basically linear. What that means is for every pound increase there is in a weight, there is a X velocity decrease in the speed at which it can be moved at. What is cool about this relationship is that certain percentages of your one rep max correspond to certain bar speeds. So, knowing this we can measure bar speed to better determine the percentage of one rep max on the bar. Why is this important? Well, we know day to day our fatigue levels change and our one rep max actually goes up and down, at least by a few percent in either direction, depending on how we feel. So at times, 200 lbs might represent a percentage of one rep max (80%) when you are fresh, but when you are tired, 200lbs actually is heavier relative to your reduced output, so now it's actually 85% of your one rep max. Any lifter knows there is a big difference between 80 and 85 percent of your one rep max. Therefore, by measuring the speed of a movement, we can make sure that the speed and percentage of one rep max line up, allowing us to train at an optimal percentage every time.