Instead of a layer of graphite pencil, she uses a layer of light-coloured chalk pastels with another layer of coloured wax crayons over the top. When you lay a fresh sheet of paper on top and draw on the back with pen, colour transfers to the other sheet.
My first attempt was a tree. (What a surprise!)
Here's the pen drawing side...
...and this is the transfer side. The result, if all goes well, is a positive-negative effect.
However, I learnt there are a few tricks involved. First is not to layer the colour too thickly. What happens is you get transfer but you can't see where it's disappeared from the coloured surface.
I also learnt that you need a distinct difference in tonal value between the chalk pastels and the crayons. My second one (above) had colours that were similar in tone, so the result was fairly ordinary. I think the best results come from using very light chalk pastels and stronger coloured wax crayons.
I'm not sure how useful this technique will be for me, since I don't do much on paper. I wonder whether the same technique would work with fabric crayons and cloth. An experiment for another day!