/3/ - 3DCG

>>389244
Or here, I'll post a picture.

Inverted normals are what allows for this effect. Maybe there's another way to do it I don't know about, but it seems like the best technique to me. You can see that the mouth appears to be farther in than it's actually modelled, and it's viewable from all sides. These are pretty key concepts in the anime and manga industry.

For example, if you look at the OP picture, you can see that her mouth is wayyyy farther back than should be physically possible. Using inverted normals it's not too hard to accomplish, at least in theory. Just move the mouth back.