Cin's quite right. The poser models are capable of some fairly extreme contortionisms. One thing I try to do when I'm working on a difficult pose and try to get into it myself: observe how my bones go relative to each other, how they got that way, what parts are forced into different places by the pose (the only reason for shin side to side, as far as I can tell!), and then try to recreate that with the model. It helps to do some sketches on paper for reference. Though if you're doing butt poses, you might want to draw the blinds. The neighbors may think you're a little weird. ;)