The problem is that two of the creature morphs, that for no neck and that for removing the middle fingers, will look weird if the joints (neck or fingers) are bent. To get around that the rotation parameters are multiplied with the applicable INV parameter, which is one on its own minus the value of the morph (three fingers or no neck), so that as the morph is turned up the rotations become more and more restricted. If the INV parameters are zeroed then the rotations get multiplied by zero, and so the joint is effectively locked.