For me it is post orgasm play or punishments. I've actually done this a few times in a CNC context (private play rather than a Pro Domme session) so I can say I love the fantasy but hate it while it's happening.

Strangely though, not long after experiencing it I'm drawn to the idea again....even though I know it's definitely in the "Be careful of what you wish for" category. .....very confusing.

As to your question about scenarios that are just too complicated or impossible I guess longtime (weeks not hours or a day) confinement featuring intense training/conditioning. Great to fantasize about but one of those things that should remain a fantasy even if it became possible.