I will try to explain it as best I can but I am no expert. That was the first time ever that I had tried dry brushing any toy as I was previously afraid of stuffing it up.
Dry brushing is used to bring out molded detail in plastic by having a darker shade of runny paint to outline surface details. It is also used to provide the illusion of 'weathering' in plastic models.
Dry brushing requires a special 'watery' paint, the little paint tub I have is labeled 'Citadel Washes - Badab Black' and can be bought at Games Workshop and other hobby places. The one I have was kindly given to me.
To apply it you simply get some paint on a pice of paper and with brush, smear it all over the surface that you want to bring out details. The paint will go into all the little gaps and lines causing them to stand out as you can see with Bludgeon's nose and teeth. The rest of the paint can just be wiped with a piece of cloth or tissue if you don't want the weathering on other surfaces.
To do my Blood head, I just smeared the black wash paint all over the mouth and nose and then I quickly wiped the paint off with a tissue removing the surface paint. Only the bits that had gone in the detailing such as the teeth and nose remained bringing out the sculpt detail.
It was all a less than 3 minute job, just be mindful that the paint drys quickly so wipe away the excess right away.
Here is a better written description by an expert:
http://www.scalemodelguide.com/guide...Drybrushing-1/