Their under-the-chin anchor is called a target anchor. Has been around forever for long range shooting. It in effect lowers the rear sight. The lower anchor also orients the bow/arrow in a more upward trajectory, enabling the archer to get more distance out of a particular sight setting. I believe Olympic shooters are also limited to one sight pin. Because Olympic bows are fairly lightweight and the shot distance is long, it would not be possible to set the one pin low enough (it would either be below the arrow or retard the arrow's flight) if they didn't anchor under their chin.