The method is proximity based, using a volume select modifier, so as soon as the lips open past a certain distance, whether by jaw rotation or morph target deformation, they will “un-stick”. Naturally, once that point has passed the threshold, the surrounding points follow suit and create a sort of peeling effect away from that initial separation point. This is the case no matter where the initial separation occurs. If instead of rotating the jaw straight down you were to rotate it to the side, it would initially “un-stick” from the corner of the mouth where the separation point is the greatest and then gradually grow from that point out. (I was actually able to create a cool looking cartoony “rolling effect” with the mouth, very similar to the three-eyed aliens in Toy Story when they go “oooOOoooOOooo”, using this sticky lips technique and rotating from side to side).

Of course, when using jaw rotation the farthest point front the rotation pivot will always travel the greatest distance. This makes the “un-sticking” occur at the front a lot more often since it is the farthest point from the pivot.

I really hope you put up a tutorial or just some clues as to how to achieve the effect!

-Julz