Nubuck, which features a pile finer than suede, is very delicate and will be ruined if brushed with a traditional hard-bristle suede brush. The softness of the natural crepe combined with it's sticky texture makes it perfect for cleaning this delicate leather.
Best used when at room temperature so that the crepe rubber is soft. Use like a normal brush. You will see that the crepe becomes darker as it accumulates dirt from cleaning the nubuck or suede. This is normal.