Scumbling is a painters technique where you apply a typically thin layer of paint on top of dried paint, then rub enough of the fresh paint off after it's applied to leave a residue behind.
Haven't seen that term used in mini or even terrain painting before, but I actually do use that technique during weathering sometimes. Never thought of it as scumbling per se, but it is what it is.
I would paint something like a rusty metal color, then when it's dried I would paint over that with the color of paint that I want to look like a chipped paint effect, say, yellow, then I would wait a bit for the paint to set up just a little and take a soft sponge and rub off the yellow paint. What this does is leave behind the yellow paint in the recesses, where naturally it wouldn't have rubbed off due to wear and the environment over time. That sort of thing.
That's the way the oil painting books I have describe Scumbling, the two I checked agree that it's similar to Sgraffito, except you don't scratch at the paint to remove it, but simply rub at it. Now, on the web you'll read a few variations of the technique, where scumbling is like our drybrushing almost, except you use thick dry paint and just trough it on and use a palette knife to spread it over a layer of dried paint...or I've seen it described as a glaze type of affair.
At this point I would say it's one of those techniques that means something different to many people. I was always taught that it was applying paint, then rubbing it away leaving it's residue behind.
Google image results have some images of the various ways people use this, that should help understand it's uses.
