See RobA's tutorials for more details, but Colors->Threshold. Basically, it turns your image into a straight up B/W. You can play with the sliders to make it more or less in either direction depending on the levels of color intensity(ie, light/dark) of the base layer. The more (wildly)varied the original image is from pixel to pixel, the more your threshold slider adjustment will break up the image when moving from black to white and visa versa. Light colors get white faster, dark colors turn black faster as you move the slider. Or, at least thats my understanding. Playing with RobA's coastline techniques or his Regional map tutorials area good way to get a basic understanding of what threshold does.