Jigsaw sketching

I know I'm not the first person to do this, but it's a great exercise for artists I think, I don't know if it has a name but I've dubbed it Jigsaw sketching. What it entails is starting off a sketchbook page with any drawing you like and then using it's negative space as the outline for your next drawing until you've filled up the page. The reason I find it useful is that it takes you in unexpected directions and challenges you to draw stuff you wouldn't normally, as well as pushing your sense of 2d design and how you interpret shapes.
Here's an example, I started this one off with the big Zombie:



Hope you give it a try, it's a lot of fun!