There are different types of masks, and variations on each type. Generally when one refers to a traditional unsharp mask, it is to a slightly unsharp, relatively thin, low contrast positive. When sandwiched with the original negative, it lowers total and local contrast, requiring an increase in paper grade, which then expands local contrast in the print. The unsharpness combined with the higher paper grade also creates edge effects which enhance the sense of sharpness and line detail.
Originally Posted by markbarendt
Masking is a broad, relatively involved topic on it's own because there are so many variations. Thinking about it though, I think tone reproduction diagrams for each basic type could be an excellent addition to some of the good masking texts ou there.