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First of all your model needs to be grouped correctly. You need 3 groups in the head: head, jaw, and jawhinge. I prefer to do all my grouping in Wings rather than with Poser's group editor. Loop Cut the jaw away from the head. Then, select another edge loop behind the jaw for the hinge. Loop Cut that too. Name the objects appropriately.
After that, go into the Setup Room. Create a bone for the head like you normally do, and name it head. Then create a bone for the jaw-hinge, then one for the jaw. Give the bones the same names as your groups, and make sure your groups and hierarchy are correct. Then exit to the pose room.
Now you need to set the joint parameters. You should only need to adjust the xRotate (bend) paramter, jaws don't need to twist ;) Set up the joint similar to what I've done here, so the upper jaw and head don't deform when the jaw is moved. It doesn't matter what settings the hinge has, because you're not going to move it at all. It's just a place holder in the hierarchy for the jaw to parent to. If you don't include the hinge, the jaw will not behave properly. The only exception to this rule is for objects that join at right angles, such as the Millenium Dragon's jaws. Also not that this method will not work well on figures with cheeks, such as humans. It works best on creatures with long jaws, like wolves and crocodiles…and dragons ;)