they work when they're turned on, unless conditions aren't right - in which cast they are either irrelevant or a hazard.
does that answer?
seriously, only use your fog lights in foggy conditions, and then do not use your high beams. If it is really bad, don't even use your headlights at normal.
If you use them in clear conditions, you'll piss off oncoming drivers.
the idea is the fog lights are positioned so low they do not reflect from the fog back into your eyes.