Release the mouse button, the shape is drawn. Now, to see how the masking works, click on textures button, select some texture and press OK button to close textures form. Change Color choice to Texture aligned Press Mask button, red color in our bumped drawing is replaced by texture. Now select some other texture. Change method to Equalize Press Mask button.
Previous texture is equalized with new texture.
Now try to apply some filter, noise, pixels shifting or any other effect on the same shape, again by pressing Mask button. This is because all actions except drawing would bump again already bumped shape. However, you can try to apply some filter with bumping on, as also nice effects can be obtained.