Its been years since I did any scatchboard work...so from memory I'll try to answer you question on it. You should be able to get a blank scratchboard from you local art supply store (if one is in your area). The technique is using a tool that removes, carves out the black top layer to expose the white underneith layer. There are special tools for it (can't remember what they are called) but some use X-acto knives or simillar tools for it.
The technique makes me think of pin and ink with cross hatching, stipling etc. techniques. Some also use watercolour or airbrush once the carving/scratching part is done. It gives a nice effect to the work at the finish.
Thats all I can remember about it from the top of my head....I haven't done any for the last 7 or 8 years...so I hope I haven't given any incorrect info on this topic.