Do you have a source? I've actually tried to look up info on this many times and failed, but was told by someone on this board that the start was also laser timed.
Here's what I know: The digital video that they use these days has a running "timeline" in the background, and the ability to insert digital tags. This allows multiple cameras to be synced together. The laser could be used to insert a tag in the video at the official stop time. They could subsequently go back in the replay, watch it in slow motion, and manually insert a tag at "first flinch." The time between the tags is then your official time.
I don't know if this is the method they use, and I don't know the accuracy or tolerances, but I do know that it's plausible. Regardless, much less room for error than a guy with a stopwatch. Any thoughts?