The tool was created to remove cast shadows and ambient occlusion from 3D models by letting users paint on the models to mark approximate areas that are lit or in shadow.
The app is said to use “rough illumination maps to perform binary classification of texture pixels into lit and shadowed groups.” Please note that there’s no need for brush strokes to be precise and you don’t need to paint carefully around boundaries. “A good markup implies spatially uniform strokes, one stroke per material which is undoubtfully lit/shadowed”, states the team.
The de-lighting tool is optimized for 8-bit JPEG compressed textures. You can get more details and find a tutorial with several examples here.