A watercolor blog.
Back runs are the "blossom" or "flower" produced when part of a wash is too wet and the pigment is lifted up and forced to gather at the leading edge of wetness. Some people try to avoid them at all costs. I happen to like 'em.
Here are two demos from watercolor class. The object was to show the traditional method of painting a wet into wet sky and then adding a blurred treeline behind the houses. After the blurred tree line dried the crisper parts of the trees were added for variety's sake.