Bending issues

I’ve been messing around with some models that I’ve found, posing them ect. ONe thing I noticed though even on some the best of models is the backside when bending foward, like touching their toes. It looks as if a bear raked them with it’s claws!! Does anyone have any good sugestions on how to fix that?

maybe add a butt bone, and/or try to fix it with weight painting. there is also the corrective shape keys add-on that comes in 2.58. ( haven’t tried it, but it appears to work well from the video )

Seen any links to how use the new feature?