Of course I have no actual knowledge but...
The way I have it figured is, it only draws the back-sides of objects if you apply transparency in the map file. But it does draw objects hidden by the object in front.
here is a related link. I think the lack of response proves my theory.
I'm still on a slow computer, and start losing FPS instantly when something isnt right..
so the way I tested was I created a worst case scenario map, where I looked thru twenty semi-transparent objects simultaneously.
If I used transparency in the matrix, FPS immediately went waaay down.
If I used a transparent texture, FPS was the same as a regular texture.
The only explanation I could come up with was that the client draws back-sides of an object only when transparency is applied mathematicly. It doesn't know the difference between a transparent texture and a regular one.
So this means it draws the sides of objects towards the player, whether another object is in front of it or not.
I'm pretty sure you will see vast improvements in occluder operation in version 2.1.