Converts a black and white 'scan' -- or other linework without transparency, to a layer with transparency corresponding to white in original image and black to solidity, with intermediate brightnesses translating to intermediate levels of solidity of black.
In short -- converts white -> transparency, black -> solid, with full antialiasing. This is a one-step process, no choosing of parameters is needed.
Also locks the alpha of the lineart layer so that it's easy to color the lines.
You are then expected to do your coloring on layers beneath.