Hi Blunt,
It's not something that people can intentionally do. Your system gets updates from the server at a rate anywhere from milliseconds up to several whole seconds, depending on net status. During those periods where your system is not getting updates, it extrapolates the last known directions, attitudes, speeds, etc, of the cons in your view.
So if you dive at the ground, and pull up quick, you didn't go underground on your end. Someone watching you may not be getting quick updates, so they will see you go underground momentarilly.
My words may not be the most accurate, but that is the general explanation.
<S>