The human reaction time is about 0.1 seconds. They shouldn't use a manual chronometer to measure these results 4.9+/-0.1 is the difference between a world record and a good score.
I believe WCA officially uses StackMats for the timing. Timer starts when you lift your hands from the touch pads and stops when you return them to the pads.
This isn't perfect either but it doesn't rely on someone else's reaction time.
This isn't perfect either but it doesn't rely on someone else's reaction time.