Yes Bill, post 28 does make sense-- and it points out the excess baggage we carry around with our choice of words.
I'd say that "Joints are flexed, weight centered, muscles relaxed and poised to react" is being stacked. The tennis player can relax his muscles more, since there are no external forces acting on them , they are just upright and "ready". But notice what happens at the moment the opponent serves the ball. The player, once rocking from side to side experiences a general stiffening and the rocking momentarily stops -- co-contraction! The CM stabilizes over the feet and can be moved in any direction.
The notion that co-contraction stabilizes the joint makes it sound like you should be a statue. However, co-contraction also occurs when making fine adjustments -- eg. like when threading a needle, your hands and fingers do more than just push - they aim. They stabilize the thread and needle as you can move the hole of the needle overtop of the stationary thread. So, co-contraction does is make movements themselves very accurate. How?
Let's get back to skiing, where co-contraction will assist in accurately bringing the feet under the CM or, the CM over the feet. (Do you move the thread or the needle?)
I think we all notice some increased tension within our bodies when we ski on very stiff skis. That can be attributed to co-contraction trying to keep us in the sweet-spot. This is no different than the "relaxed and ready" tennis player moments before impact of the serve. To my mind co-contraction resists overshoot, and transmits movements at the feet to higher joints in the body, which enhances proprioception. This has an immediate effect on your automatic balancing moves, as a result your balance improves.
Another cheesy example.... In a deli you pickup a piece of cheese. How heavy is it? Do you hold the cheese motionless at the end of a very tense arm, or do you hold it gently, moving it up and down?
In the motionless case, you can tense so much that the weight virtually disappears -- you've short-circuited your proprioceptive system.
In the more gentley held (dare I say "relaxed"?) case you're creating a functional tension that allows you to sense the weight of the cheese more accurately. In comparison to full-on arm locked tension, the more "relaxed" case does have some tension. It was created by co-contraction.
Ever hear the direction to "activate the core" when skiing? Abs and back pulling against each other? That's co-contraction. It lets you accurately detect and react to movements in the spine/hips.
At least, that's how I understand it to work. So to me, "stacked" is equivalent to :
"Joints are flexed, weight centered, muscles relaxed and poised to react."
When "muscles relaxed and poised to react" means "muscles in a state of functional tension". The method to develop that "functional tension" is "co-contraction".
Does that all make sense?