Forced subtitles are just that... forced.
Even if you don't set the subtitle to ON, it will still display.
A perfect example is District 9.
If you play it back using a Bluray player, you will get subtitles during the portions of the movie where the aliens are talking EVEN if you set subtitles to OFF during setup. That's because the movie experience is affected without them.
The problem stems when you backup BD to MKV format.
Even though all the MKV tools allow one to rip the subtitle tracks and even identify them properly (default, language, forced), currently the firmware doesn't detect the forced subtitle flag (in mkvmerge).
So using the current firmware, if I watch District 9, I have to turn subtitles on manually during the alien parts if I don't want to see english subtitles at the same time.
Other movies that use Forced subs;
Avatar - Navi speaking scenes
Kill Bill Vol 1 & 2 - Japanese, Chinese speaking scenes only
Inglorious Bastards - German speaking scenes.
Bourne Trilogy - foreign language only.