Well, it's a combination of reducing body fat and muscle growth. When you are at low body fat, your cheekbones and jawline become more defined. But you also have muscles in your jaw, which can be worked just like any other muscle in the body and give a more square-jawed appearance (think Brad Pitt, Guy Pearce).
The combination of the two can give your face a firmer and more masculine appearance (so it's a good way to get rid of a "cheeky", "cute" or "nice guy" look).
I think there is also some relation to the increased testosterone, but I'm not sure what effect this has on a person's features.