Stumbling is a method by which wet paint is dragged over dry oil paint to create a different painting effect. The wet layer should be an opaque paint application ( not be be consumes with transparent glazing). The idea is to let some of the underlying paint color show through.
If a color is stumbled over it's compliment you can get a beautiful mystic optical gray tone, which is side for landscape chiaroscuro effects.
Here is a great vide by Winsor & Newton that explains it all.