He doens't say how to do it in his thread, just said he used blendshapes. I've never done this before, so I can't really help you. You should ask him yourself. An idea I have to approach is to just model the crack first, just to see how it looks. After you finish it, put it in a layer, use it as a reference, then build yourself the model you referenced one at a time, limiting yourself to the amount of geo you used. I think that sounded a bit confusing, so you should ask him yourself. Wish I could help.