Here's something that might help you get a feel for it. A modified Poser box prop that has most on the normal channels removed for the sake of simplification.

Load the Test_Box in Poser, use only the very bottom yTran dial, and then try the zRotate dial. Note that the rotation works as per normal. Now Restore the prop (Ctrl+E), the two extra yTran dials will now have none zero values. Try the zRotate dial again, note that the prop now rotates round a different center. Now swap the sign of yTran_A and yTran_B, note that the rotation center has changed again. This is the same thing that happens with the chain, only a lot of none essential stuff has been striped out so that you can see the nitty gritty.

The value of yTran_B defines the offset for the center of rotation relative to the props normal origin, the value of yTran_A should always be set to the inverse of yTran_B.