GIMP :: Effective Shadows Technique

I've tried doing this on similar close-ups of my own characters, but I can't get it as good as done here: I know it requires layers, and the smudge tool to get the results. But what is the technique used here where the lines are like 'marching ants"?------is this simply the 'magic wand' tool? I also reckon that Gaussian blur is used here also.

The steps are not made clear on this very effective technique---also the GIMP typeface/controls are not in English, but hopefully some good folks on here can recognize the style used here.....it's just the sort of thing I'm looking for to bring a bit of depth to my cartoon character shading.

I'd love to introduce this technique [ So far, I haven't been able to get the the dodge/burn tool to work as effectively as some other GIMP users can, but I'll certainly return to it].