Why do athletes sometimes dip head when finishing race when clock does not stop when head crosses line?

The time you post for a race is measured as when your torso crosses the line. Hence, the dip can be the difference between two athletes, who have run just as fast as one another, finishing ahead of the other. A runner who doesn't dip would have run a 'faster' time had s/he dipped (with correct technique of course). A common problem is when a runner, out of desperation, dips for the line too early. This can cause a significant impact on running technique, and result in a slower time than if the runner hadn't dipped, or certainly dipped at the right time.