My first winter session at the Barn started Thursday. I wanted to do something with fog and mist.
First we talked about what it means for "more moisture to follow less moisture." That always confuses me. So I think of it as a sponge. If you have a puddle to clean up, would you use a dry sponge or slighly damp sponge to pick it up? Answer: a slightly damp sponge will wick up the puddle quicker than a dry sponge. The area of more moisture--the puddle--is drawn into the less moisture of a damp sponge. It will resist the dry sponge. (And yes, my students said they'd just use a towel.)
If you have too much water or paint on the paper and need to pick it up, you have two choices: you can dab it with a paper towel or tissue, which can leave the paper with a hard edge, sometimes soaking up all the color. Or you can attack it with a "thirsty" brush: one that has been dampened, but the excess water squeezed out of it. That will soak up extra paint and water leaving a softer edge. Both methods have their uses.
Another reason to remember this is when you are trying to create a smooth wash. If you have wet the paper, the paint is going to be attracted to the less wet...but not dry...areas.
If you are getting (or want to get) blossom effects, it is because an area that is wetter is placed next to an area that is drying but still damp, and the wetter area is trying to move into the damp area, creating blossoms.
Also, this concept is used to create soft edges in things such as fur, shadows, sunsets, and fog.
This idea is necessary to keep in mind if you want to create a picture with fog, mist, or "lost edges."