There's an outstanding discussion of this going on on Lenny Balsera's blog right now:
http://lcdarkwood.livejournal.com/3824.htmlhttp://lcdarkwood.livejournal.com/3858.htmlThe comment threads have a lot of great discussion about this, so don't stop at the end of the posts.
For what it's worth, I think the key point is that in order for an aspect to be compelled in a way that should generate a fate point, it needs to have
dramatic consequences. So, for "HULK SMASH!", a good role-playing moment might involve you shattering through a door that could have been quietly picked. A COMPEL would have you break through a door that could have been quietly picked, and smash an irreplaceable art installation, earning you the public emnity of the artist or owner. Or maybe bring an enemy into the fight that wouldn't otherwise have been there in a more immediate sense. Or maybe the NPC you came to talk to is knocked unconscious.
If you just knocked down the door, great, you're the Hulk. But when you are in that position either you could hold up a fate point and say "So...I think the hulk might be smashy, here" or the GM could say the same to you. If you both agree there's going to be a compel, you're agreeing that
a) You're going to get the fate point.
b) Your character will have cause to regret it.
but note that b) is
not "YOU will have cause to regret it". You'll have more adventure hooks and things to tie your character to the setting (a new nemesis/detractor, more bad guys to beat up, a challenging puzzle to solve). And that's awesome.
I didn't really grok this as well until I read about damage in Truth & Justice. In T&J, skills and aspects are sort of merged into these hybrid "quality" things. And when you take damage, you temporarily lose value from your qualities themselves. And whatever you choose to be the _first_ quality to drop for damage generates a Plot Hook. So maybe you get kicked in the junk, so you drop your "Businessman" quality to Poor. The GM immediately notes down something like "Meanwhile, across town, Spider-Lad's secretary is mugged on the way home from work!". It's a little bit different, because it's not tied to the Fate-points in the same way, but it's therefore much clearer what's going on. I recommend reading through that system if you're looking for some more ideas (the example of combat with Winter Hawk is worth the price of the pdf). Note that this is specific to T&J, so you won't find it discussed in the PDQ core rules (shame, really).
Anyway, so Compels are sort of like that, but instead of triggering on your character taking damage, they trigger on your character making particular choices.
Hope that was more helpful than confusing. =)