If you’re wondering how so many of those kids at Georgia’s spring combine were able to turn in 40-yard dash times that would make Usain Bolt jealous, take a look at Tim Tucker’s description of how the staff measures dash times:
A measure of linear speed. The players will sprint 40 yards as fast as they can. They are timed from their initial movement through the finish line. Each player is timed by four different coaches. Each player gets two attempts, with their best time recorded. [Emphasis added.]
In other words, a player starts and then a stopwatch is clicked. In Bolt’s world, the runners wait for a signal and both they and the clock start from that. How much of a difference in time results from that?