In the woodworking hand tool context, uzukuri are stiff brushes made of tightly bound fibres of organic origin. Thanks to the use of different fibre materials, they come in 3 levels of fineness.
The primary application of uzukuri treatment aims at creating a textured and/or polished wood surface. As you rub the workpiece back and forth with the rough brush, the wood fibres in the lower density, soft grain aspect (earlywood in most species of timber) are gradually removed. The higher density, hard grain aspect (usually latewood) is also getting worn at least to some extent, but at a much, much slower pace. As a result, you raise the hard aspect of the grain and, in effect, accentuate the grain in the workpiece, 3-dimensionally.