Think it's unnecessary for poker players to wear those dark glasses when playing cards? A study by the University of Melbourne found that our eye position betrays the numbers we are thinking about.
It seems that when we think of numbers we automatically code them in space with smaller numbers falling to the left and larger numbers to the right. (for those of us who read and write this way!) We unconsciously place numbers along a left to right oriented number line.
Paying attention to where someone's eyes go when they are thinking of a desired card will give you some insight into what they are potentially holding. Yes, the movement may be small but if you've got thousands riding on it you learn to watch for small discriminating give-away movements such as this.