You're on the right track with the way they optimize, but rather than splitting world geometry, it splits vis leaves. Vis leaves need to be efficient in what they cover, or else, as in the example you gave, the entire length of a coridor will be rendered, regardless of what is in view.
In the above picture, you can see (with my crazy Photoshop skillz) what happens when a hint brush is applied to something like this corner. Before hinting, visleaf 1 can see straight into leaf 2, and vice versa, so both leaves are rendered all the time. However, after a hint brush is added in (the purple line), area one can only see area 3, and area 2 can only see area 3. So if the player was to stand in visleaf 1, leaf 2 would not be rendered. Of course, this isn't the only case where hinting can become useful. Experiment with gl_wireframe a little and see where your map needs these useful little things.
Here's a little guide on optimization. The guide itself is for Source, but the hinting principles remain the same. So >>>
here<<< you go.
That's hinting, in a nut shell