I think that, yes, my first statement didn't express the possible full observations.  Now that I have observed this for a bit, the direction up/down here is irrelevant.  It has more to do with the direct distance of the goal from the start of the IK chain and the length of the links.  So, stated more clearly:

When the direct distance to the goal from the IK chain start is less than the summed lengh of the chain links (body part lengths), the goal body part is 'planted' to its original position and rotation.  Not just the tip of the foot.

IK doesn't work like this!  You have a goal point.  You have a tip/end-effector point.  These two points are in a continual process to be coincident as the goal is moved.  When coincidence is not possible (when the distance to the goal from the chain start is greater than the total lengh of the chain links), the chain just 'points' toward the goal.  When the hip is translated, the goal and tip will remain aligned as long as that distance relationship is maintained.  In other words, the example video for Jessi - that is how basic IK works in all systems.

But Poser, with 'Use Limits' disabled it seems, the entire foot or hand (not just a single point) will remain aligned as long as the distance relationship is such that the goal-start distance is less than the total links length.

Here is the default stance:

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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