would have liked to have seen a better explanation of the red lines in the image. it sounds like those are added by humans to outline the runners' chests; how do they ensure the lines are drawn on the frame exactly at the finish line?
The x axis in the image (left right) is the time domain.
Imagin instead of the people running from left to right, instead they were still and the pixels were moving right to left, or if that breaks your brain, think about if the track was a people mover and the runners were a static diorama moving left to right and the sensor is just it over time. But also none of that is real.
It looks plenty reliable as long as the camera was positioned and calibrated correctly (which I assume was carefully tested beforehand).
They don't.
The horizontal axis is time. The red lines mark the time each runner's torso was counted as crossing the finish line.