I find it uncanny how often receivers, RB's QB, know exactly where the line of scrimmage is while running, changing direction, avoiding tacklers and diving for a first down. It's almost as if they, too, can see the yellow line. I know there are line-to-gain markers on each sideline but I find it incredible that they can see them as they are diving through a pile in the center of the field. 

Anyone who played, can you describe how it is that they do it? Can they actually spot the markers? Or do they make a mental note of where they need to get to before the snap?