Its wall of text time!
Almost a year ago, LL introduced a 'new' feature to the grid: Avatar Physics.
Avatar physics was a great feature, it took what was previously a TPV only feature (Emerald's 'Jiggly Boobs') and improved on it.
Originally, each compatible viewer could activate settings that would add bounce to the breasts of all female avatars that walked into view, regardless of whether the owners of those avatars wanted a spring in their step or not.
Avatar physics was different, it added a new class of wearable object, a new layer that sat under the skin and above the shape. Now the owner of an avatar could decide how it moved, from 0 movement (without a layer), to helium filled space hoppers. Not only that, but the new system added butt and belly movements (also available to male avatars) to the original bouncing breasts. Naturally some people cried out that there were more important things for LL to be doing than titillating tit shaking, but its here now and it adds another aspect to the ways we can customise our avatars, even to the level of varying the 'bounce' with outfits (For example a high movement layer with a string bikini, and a low movement layer with a sports bra and yoga pants.)
It's not perfect though. The Emerald (and later Phoenix) system, came with pre-defined settings you could tweak if you wanted. Like newly created shape layers (which start out looking human but are pretty plain) new physics layers don't do much and require the user to find 'realistic' settings*. By default the viewer spins the camera around and puts the avatar into the 'edit pose' so you can't see the effect of changes until you save them, and then move the avatar. Fortunately with a V2 based viewer you can start an animation playing while in edit mode to see the effect as you move sliders, In Firestorm (and possibly others I don't know) you can even turn off the automatic camera movement altogether, so you can tweak the settings on the fly, if for example, you were at a club and found you were moving in a way you didn't like.
Incidentally I find dance anims are the best for testing physics settings.
*see the end of this post for some cheat notes.
The biggest problem with avatar physics though (besides the fact it's down to the user to figure out the most 'realistic' settings with a lot of trial and error) is that it's kinda broken.
I spent hours tweaking the butt settings to give my rear the kind of short quick jiggle a well toned behind should have. But when I went to a busy sim, someone asked me why my butt was vibrating. Turns out, when a viewer (any viewer) goes below 15fps (which is quite possible on a complex sim) the physics system looses its shit and starts violently shaking in a way that will never stop (it can happen if your viewer looses focus too). It looked fine on my screen, but i had to remove the layer because their viewer was struggling and telling them I'd gone for comedy settings.
The worst thing is, other than avoiding other people (not too hard on the grid these days - Zing!) there is nothing you can do about it. You can have the best computer with perfectly tweaked settings, but if someone sees you from their crappy old box with the settings way to high, their physics will go crazy, but you are the one who looks stupid.
I've given up on a toned bum now, and opted for a more 'wobbly' and 'fatty' behind, because large gentle movements don't go mad like small fast ones, but I still have my old layers in a box, in the hope LL will one day, you know, FIX IT
A JIRA (bug report) was raised about this issue a long time ago, but it still hasn't been fixed (the Linden originally assigned to deal with it was let go as well).
https://jira.secondlife.com/browse/VWR-25545?
If, after reading my post you're concerned about this issue, I urge you to go to the JIRA above, log in, and 'Vote' and 'Watch' (you have to pick both, LL admitted they ignore issues with a lot of votes if not many people are watching).
Smallprint: If you're not familiar with JIRA's and don't like the idea of entering your password into a site you got from a link, you can also get to it from the help>bug tracker page on secondlife.com
A Guide on what the sliders actually do can be found here:
http://tali.appspot.com/html/tutorials/avatarphysics.html
*This is where I promised to post my current 'medium' slider setting, and finally I remembered!
Breasts Bounce
21 28 18 66
Breasts Cleavage
6 57 100 71
Breasts Sway
10 28 9 66
Butt Bounce
30 18 45 60
Butt Sway
24 15 36 60
Advanced
14 52 85 14 15 50 1 100 10
That's it for walls of text for now, we will return to our usual service of pictures of half dressed avatars in the next post :p