I know the advertising and specs for the v5500 mention a t-buffer. But I've never seen anything about it anywhere. There is nothing about t-buffer in the 3dfx utility either. Any help would be apprciated. Thanks.  

T-Buffer allows for the following effects on the Voodoo5:

Depth of Field Blur, Motion Blur, Soft Shadows, Soft Reflections & FSAA.

http://falconfly.de/artwork.htm
hare are some techdemos

and a screenshot
http://tdfx.de/pic/abc/antialiasing_01.PNG

So it was more or less just a tech demo? I thought it might be something like fsaa which could be turned on with 3dfx tools or something like that.

the FSAA is in the driver and works with every game

but  Depth of Field Blur, Motion Blur, Soft Shadows and Soft Reflections is only in this demos

Yep.

The additional features like mentioned would have required specific software support, so the game/demo would have had to know, when and where to apply Motion Blur for instance, or everything would have been blurred (not good!). Same goes for the other effects. For FSAA it was possible to apply it globally, as this resulted in the exact desired effect..

Another example for Motion Blur is the Quake 3 Arena Techdemo for 3dfx cards, which showed off some Motion Blur via OpenGL, talking to the T-Buffer directly via some MiniGL extension I believe (but unsure).

So actually in that Q3A Demo Motion Blur was actually emulated in a way?

No, as he said it contained native T-Buffer support via OpenGL Extension done by 3dfx.