Imagine a corner of a hallway. Your goal is to not draw what can’t be seen. So extend the shorter wall (the one that turns the corner) with a brush that is flush with the end of this wall and has a face that shares the same plane as this wall. Apply “skipâ€? to the entire brush. The face that shares the same plane as the wall of hallway, apply “hint”. What you’re saying is on this face and this face only, do I want q3map2 to recognize a hint, which nudges q3map2 to use this plane, this face, as a facilitator to not draw what’s beyond that corner. If you made the entire “skip” brush have “hint” on every face, then you’re being inefficient. q3map2 is likely to get confused, too many or frivolous hint planes will only hurt your map and its compile times.
Experiment with it, make a test map. It gets more confusing with larger environments, like my current project, ruins, a map open to the sky. Keep it simple till you’ve understood it and you can see it working. Least this is how I go about it.