Specifically it means this: At a drag strip, the car position is measured by breaking a light beam low to the ground (think where the tire is what blocks it). So when cars stage, they try to barely block the beam since the timing will not start until the beam is behind the front tire. So while the tire blocks the beam, the ET clock hasn't started yet even though the car is moving; you can also start moving before the go signal on the christmas tree, as long as you're still blocking the light when the time starts. The 1' rollout is meant to simulate this effect since any time coming from a drag strip have this inherent feature/flaw