Back Shading

I am trying to get my students to understand what back shading is (well, at least that's what I call it. Is there another official word for it?)

So while this is not EXACTLY what they were to do for their assignment, it's close. 

This a drawing of flowers in pencil with red ball point pen back shading.

Their project was to draw flowers shaded (and back shaded) on a painted watercolor surface. It was my first attempt at this assignment, but I'd say it was successful!