White line printing is based on the Japanese woodblock printing method but is totally American. While the Japanese have separate blocks for each color in their prints, Americans decided to use one woodblock but to print each color separately. In order to do that, the print paper must be lined up exactly the same each time. That is accomplished by folding one edge of the paper to create and edge that can be tacked in place and a printing surface that can be folded up and down as the artist adds each color.
Here is a better description of the process by someone who knows more than me. And below are some white line prints from Kate Hanlon's blog.