Quote - The axial twist rotation will be along a line drawn between the center and end points.

PhilC is a very knowledgeable fellow, and it not often that I disagree with him, but I'm going to have to disagree on the above statement.

I don't think the endPoint has anything to do with axes of rotation, that is to say changing the position of the endPoint won't have any effect on the rotation of the actor! The position of the endPoint may affect the way the joint bends, but in Poser rotation and bending are two different things. A rotation spins an actor round one of its axes, and is dependant only on the position of its 'origin', its 'orientation', and the order of the rotation channels in the channel stack. A bend on the other hand is a deformation applied to the mesh by the JP channels when a transform is applied to an actor.

The axial twist rotation may often be along a line drawn between the center and end points, but there is no causal relationship here. You can move the endPoint and the twist axis will remain unchanged. Likewise, if you change the 'orientation' of an axis, the endpoint remains in the same location (the endPoint icon may spin, but as far as I know this spin has no consequences for the actor).