A false start is defined as a runner leaving the starting block before the starting gun sounds, or if they start in less than one-tenth of a second. Though one-tenth of a second after the gun is technically part of the race, research has found that it isn't possible for a human to have a reaction time faster than that. Starting that fast is considered anticipating the gun and, thus, cheating.
If you do jump the gun a bit, it isn't the end of the world. Runners are allowed one false start at no penalty under current international rules, but a second one will result in a disqualification.
False starts are detected by ReacTime, a device that sits on the back of a starting block that is connected to a main computer. The device detects the pressure of the athletes foot and starts the time when the starting gun fires. The device measures the exerted and decreased pressure as a runner pushes off down to one-thousandth of a second. Nothing gets by these things!
