D3D: You have to compile the level in BSP mode. That's the first button in the Map Compiler dialog. Compiling takes longer then because the tree and the PVS are calculated. The editions below Pro ignore the BSP tree and only use ABT for rendering. Pro uses three stages, first BSP, then PVS, then ABT.

Father: there's no demo that I know of, but that's a good idea - I should indeed do one for comparison. A BSP tree is a culling algorithm, it has has nothing to with shaders or with shadow maps. You can find a brief description about what a BSP tree is and how it works in our manual under "Rendering Engine", and more detailed in the Internet.