I appreciate you pointing out this problem, it's a pet peeve of mine. At showcases, many (most?) kids jump up early and are literally striding across the plate when the ball hits their glove. When I use video to time this sequence, the advantage of the early start is 0.2 - 0.3s, sometimes more. They can still jump early on a knee high pitch, although the transition time will be a bit longer. The pop time measurement as it's currently handled has to be the lamest "objective" measurement used in baseball. It's like allowing some kids to get a running start on the 60. Even PG will occasionally acknowledge this problem with a comment such as "1.78 pop in drills, 2.1 - 2.15 in-game pop". I have to believe that college coaches roll their eyes when they see this stuff. Perhaps putting a batter in box and taking the measure on a swing-and-miss (or in-game pop) could standardize the measurement.