He's had far more practice at phrasing things better.

You forgot: "Sweaty tree skiing"

I think CM probably changes regardless of lean or lower leg angulation on the slopes, even if its slight. On the carpet you may be able to contort yourself so that it may seem like it is not so, but while skiing you'd probably not want to sit back, and even if you were sitting back a bit and thought you were just flexing your lower leg forward, my feeling is in all likelyhood your upper body is reacting as well and overall CM is changing. if you are sitting back enough, then no your heels would stay put largely although -- again it seems like to me there's some unloading on the heel -- and this may show up at the binding. If you are balanced over the skis, i'd guess there'd definitely be some unloading of the heel - you know i have thought skiing was more of a ball-of-the-foot sport although this is somewhat counter-intuitive ... Thinking about quick changes of slope towards more steepiness, you'll almost always pitch yourself forward enough to unload the heel some, to quite a bit... I think this is definitely true for groomed slopes where shovel bite can be aggressively applied. On powder, hmmm ... my experiences are limited enough that i can't even speculate very well, i've never really skied in bottomless powder at high speed, only about 2-3 ft with fairly firm snow below it... i only remember letting the skis turn and not forcing anything like i tend to do on groomers, still there is an unload between arcs. ...just trying visualize it all...

KevR - most binding allow for some elastic movement in more than a few directions. While Rossi turntable binding are pretty solid on the heel the tow allows for some elastic defection. Marker allows toe and heel deflection before release. The heelpiece may lift a tad, but you are probably ok because latterally your heel is not going anywhere. a 5.5 setting is not that heavy so even though the heel spring is preloaded, it will deflect a little .. it's not like you are running greenspring bindings. You should be able to lean waaay out over the tips of your skis like a bowspirit and not have a release.

I personally crank my heel piece DIN setting ~1.5 higher than the recommended setting (at 140 lbs and a III+ it is usually around a 7,5-8 but I put it on 9 usually) but that is just me and I NEVER recommend it to anyone. On my big-mountain skis I do set the binding to 10 because in most situations it is better they stay on than come off, again I would NEVER recommend it unless you are doing a lot of no-falls stuff.

But definitely, when in doubt, have a binding tech do a load test on 'em.

The "play" you refer to in this post is called elastic travel. All brands of bindings have some built into the toe and heel piece. In fact, there have been performance arguments about how much elastic travel a skier should have. It is true that your height, weight, age, skier type AND boot sole length all account for the selected DIN number. The travel acts as a shock absorber of sorts, this way as you torque the binding (either by twisting or upward load) you have a little give before releasing from your bindings. It's so the things don't act like a light switch either being on or off. As most everyone has mentioned, let a certified tech at a shop set and test the system with your boots.

I will probably get my bindings checked just to make sure they are set the same in real-force... i turned them down from whatever the original recommendation was for my size and ability a few yrs back. basically i've a screwed up knee, so i thought -- hmm, well if i never pre-release, ... anyway, no problems so i kept them light.
however now i realise that maybe they can be too light though. i am thinking the binding elasticity or whatever you'd like to call it adds a little hysteresis in things... maybe a bit tighter would be better! More responsive right? anyway, a little experiement but still it will be on the light side for me I think.