This is an example map on how to make peepholes, a great new thing. To use the peephole in this map, walk up to the crack in the wall and press e, the action key. next will be your view, peeping through the crack. You can use this skill for doing hotel room peepholes, peepcracks, and even the looking thing on top of a subwarine.
It's called a periscope
Where would someone put this in a map?
BTW, are you a very bratty person?
The CZ effect is something like the fiber-optic camera in Splinter-cell, not a mapping trick. Is just the mapper (or level designer) that decide when and where the player can use the f-o camera.