Please explain "forced subs" to me

When I am putting a subtitle track into a DVD using muxman, I see the option to mark it "forced".  What exactly tells the DVD player to display the forced subtitle?  Does it just automatically happen when the language marker of the forced track matches the language marker of the current audio track, or does it have to be selected via some menu logic?

How do you mark a subtitle as forced for HD playback?  I see no such option in tsmuxer.  Does some other software do this?  Free, preferably.

Bah, apparently marking it as forced does absolutely nothing--at least for DVD--you would still need to use some sort of menu logic to turn them on and off when you changed audio tracks.

MultiAVCHD looks promising for HD encoding, perhaps they work differently, although my hopes are low at this point.