Basically, detail brushes do not block VIS.
Here is an example (pardon the ****ty ASCII):
^^^^^^^^^^^^^^^^^^^^^
^ ___ _______ ___^
^ |room w/ | ^
^ |stuff |stuff^
^^^^^^^^^^^^^^^^^^^^^
^ = structural
| _ = detail
In this case, every room would be rendered at all times... the walls being detail, the engine would still believe the player could see through them, even though they are solid, and the player couldn't.
Anyway, this tutorial would probably explain it better:
http://www.nibsworld.com/rtcw/tutorial_detail_and_hint_brushes_part1.shtml)
:p