What's dry brushing?

The term is somewhat contradictory, but at the same time pretty close to correct

You put a little bit of paint on something like a coffee can lid, touch the brush to it, and then wipe as much off as you can. Then just barely brush the paintbrush over the surface and leave a little wisp of paint behind. If you are drybrushing raised edges (i.e. an instrument panel) and have too much paint it will flow down into the instruments and ruin the effect. If you have too little paint you just make multiple passes until it's the way you want it.

The reason that people talk about letting it dry is because they use artist's oil colors which take a good while to dry. Acrylics are to quick drying to work well. Enamels work better.

Later ... Here's a link to a good article on the technique: http://www.finescale.com/fsm/default.aspx?c=a&id=287

I'm not very knowledgeable at the technique, but those are the basics. I've tried it a couple of times and had good results and bad results. It takes some practice to get it right.

For me the best way (and the way I was taught) is to barely touch the tip of the brush to the paint and lightly scrub (brush) the brush across an absorbent surface (a 3 x 5 file card works well) until only a little color remains. Then apply as Scott describes. And he's right, you need to practice.