I was using this technique long before I learnt the name of it. Sgraffito is a drawing technique where a sharp tool is used scratch into a surface to produce drawings with fine lines.
Sometimes sgraffito is used as a painting technique as well and this is how I have used it. For example you could be painting a portrait and use sgraffito to add some precise detail to the hair. You could apply for example a light layer of paint on a dark ground/background then scratch into the wet paint.