You're right, Fate doesn't have triggered actions... so defending isn't an action in the sense that it consumes a limited resource. You do it when you face an attack, whenever you face an attack, however often you face an attack.
All the "defend another" choice does is change who's at risk and making the roll. If you jump in front of a bullet for someone, the narrative says that now you're being attacked instead of that person: simply by declaring that you're in the way of an attack, you get to defend against it. If there's a catch or a limit to this choice (beyond taking the fallout if your defence fails), it's this: you have to be in a narrative position to justify taking the bullet. If you're too far away, for example, you just can't do it.
Here's a (slightly abridged for clarity) example from one of my games a few months ago.
Zexpyralis's player starts his turn. The GM suggests that, given his aspect My wrath knows no bounds, it's reasonable that the cyborg dragon would take offence at what the dwarf king just said. Zex's player agrees to the compel and takes the offered Fate point. Then Zex uses his once-per-session Fire Breath stunt to attack the entire dwarven delegation/army. The negotiation is about to turn into a war. The GM picks up his dice because being targeted by an attack means the dwarves will defend against it.
Vogue's player decides that, as a Knightly telekinetic under orders from his boss to keep things peaceful, he's going to have to stop this. He tells the group that Vogue (who is very near Zex) jumps in front of Zex and uses his Force skill to roll defence on behalf of all the dwarves at once by containing the fireball in a force field. He's going to take a LOT of stress, but it's in character and he's built for this sort of punishment.
The GM puts down his dice because Vogue is taking the attack instead of the dwarves. Zex rolls his attack and Vogue rolls his defence. When the smoke clears Vogue has some nasty consequences from the effort of containing such a massive fireball--but nobody else got hurt!
That's the end of Zex's turn, so the party's Stone-faced diplomat steps up next to convince the dwarves it was all a staged performance, and the king is very impressed.
As you can see, the mechanics were driven by the narrative of the scene and the whole thing happened on one player's turn. Had Vogue been too far away to justify using the Force to contain the fireball, the other players would have called him on it and he wouldn't have been able to do it because it didn't make sense. If you want to talk about it in terms of triggering and actions, Defend is a free action triggered by being in the way of an Attack action.
