Originally Posted by drakulie
It takes the reading as it crosses a certain plain/bubble in front of the radar gun. So, it has to come into a ceratin area for the serve to be clocked. I guess the answer to your question is it does not measure it immediately after contact, rather as it approaches a "that area" in front of the gun.
So really, some of those serves could be in the 110s. Nice. Although, as some others have commented, it just doesn't look that fast, but I realize that it's a low quality vid and would be much faster in person.