The man – in a suit with a briefcase running along the front of a wall – is unaware that he is approaching the edge of a cliff.
Positioned just behind the wall, in blue jeans, is a guard. His job is to prevent the man from falling over the cliff, but he will not see him until he reaches the end of the wall.
In the image to the right, there is a bright sunlight behind the runner, making him cast a long shadow.
Upon seeing the runner’s shadow, the guard detects the runner earlier, giving him more time to react, improving his chance of successfully intervening.
Earlier detection -> Faster Response -> Improved performance.