To turn on "true ambience" you have to set your antialiasing level to "premium quality". This will allow you to turn on a number of render abilities such as "true ambience" in the render properties panel. It'll also allow you to change your rays-per-pixel setting...which you'll want to increase to keep these effects from looking too grainly. That also increases render times many times over.
First off, true ambience affects the ambient channel of your textures. It looks best with the ambient level for your textures set to about 25% or 50% of the diffuse channel. That means, though, that Bryce's normal ambience will horribly lighten your objects unless you "turn it off" by setting the ambient color in the sky properties to black. Unfortunately, that means that the old way of making glowing bits, like windows on a ship, through the ambient channel can not be used effectively with true ambience.
Then, true ambience looks for existing light in the scene, meaning if you have a light shining on an object, or a sky, or glowing volumetric objects (which can glow independently of the ambience setting), they will appear to cast light - radiosity (or "true ambience" here) simulates light bouncing off of surfaces. It also takes reflection and refraction into account on the bounced light. A bit more on that later.
Something to keep in mind; the level of "true ambience" bounces (meaning, the number of times diffused light will keep bouncing off of surfaces) is tied to the ray depth. You can find that by the shadow options in the render properties panel. Turn it down unless you want very long render times...even setting it as low as 1 works decently. Unfortunately, that pretty much precludes including anything transparent in your scene, as the ray depth (which usually controls how many surfaces a ray will pass through before dying) is so low.
Now, when I said that "true ambience" takes refraction and reflection into account, that's only for light that's already bounced off a surface. You can light a scene by creating an object with diffuse properties only, and shining a light on it, and having the rest of the scene only have ambience in their materials. If your ray depth is up a few levels, the bounced light will be refracted through any glass-like objects you have and reflected off of any reflective objects, showing true caustic effects (the bending and reflecting of light). This takes a LONG time to render, though.
I'll just point out here, again, that Bryce lights only show on the diffuse properties of a material, and "true ambience" shows on the ambient properties. That means you'll most often want your diffuse and ambient channels to use the same textures, but set the ambient channel to 25%-50% of the diffuse channel. It also means that, unless you want a horrible amount of regular ambience in your scene, you have to set the ambient color of the sky to black, effectively turning it off.
Personally, I'd like it a lot better if "true ambience" affected the diffuse channels, but...it doesn't. 
Quick tip: one of the basic uses for "radiosity" in CG is to show off a model by lighting it with a "sky dome". To do this quickly and easily in Bryce, make a scene with just a ground plane and your model. Set the model's textures to use the same textures for ambience as diffuse, and set them to the same levels (unlike I advised before). Edit your sky to disable the sun (and the visible sun glow), shadows, ambience, haze, fog, clouds, etc. Change your sky type to "custom" and make the sky a solid color (white works well). Enable "true ambience", set the ray depth to 1, and turn up the rays-per-pixel a bit. Render.
(Of course, if you want to get creative and include a full sky, that works too, but usually people use a solid color for these types of renders)
There you go, a basic guide to using "true ambience". You're welcome. 
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(This message has been edited by Weepul 884 (edited 04-25-2003).)
