There's actually multiple answers to that question in Fate Core and The Fate System Toolkit, but they seem a bit contradictory. I'd like your help to sort them out.
Note: I know characters can indirectly defend others by creating advantages that hinder enemy movements and attacks or help allies defend, but the question isn't about that.
Here's the first answer (FC159):
You can even make defend actions on behalf of others, so long as you fulfill two conditions: it has to be reasonable for you to interpose yourself between the attack and its target, and you have to suffer the effects of any failed rolls.
Okay so I'm thinking this is pretty flexible, a combat oriented character can take the hit for another if they're close. As long as someone is taking the hit, it doesn't matter who, mechanically.
Then there's the second answer that confuses everything (FC160):
If, for whatever reason, you want to forego your defense and take a hit (like, say, to interpose yourself in the path of an arrow that’s about to skewer your friend), you can. Because you’re not defending, the attacker’s rolling against mediocre (+0) opposition, which means you’re probably going to take a bad hit.
Now it sounds like if I defend instead of someone else, I don't get to use my skill. Actually I don't even roll at all, I get a passive defense of +0. That's less flexible.
But then again it says "if you want to forego your defense", so maybe it's a way to defend someone when things have already gotten dire, not the way.
Then there's the 2 Fight stunts in the Fate System Toolkit: (FST34, 41)
Ally’s Shield: You can invoke Dwarven Shield-Maiden when a nearby ally suffers an attack. When you do, redirect that attack to yourself. Your defense is Average (+1) against that attack.
Defend the Weak: When someone is attacked physically while within your zone, you can spend a fate point to redirect that attack to yourself. You may defend against that attack at +1.
They're basically the same: It costs a Fate point, you roll defend at +1, you suffer the consequences. The stunt seems to suppose that the default option to defend on behalf of others is with a passive +0 defense.
In other words, for a Stunt and a Fate point for every use, I can get a +1 to defend others, and a roll that can worsen or better it. Why wouldn't I just invoke an Aspect for their defense or mine if I interpose myself, gain a +2, and save the Stunt for something useful?
How does defending on the behalf of others works in Fate Core?
If I'm not reading this wrong, the defending stunts are not very good. So, how have you made defending others work in your games, or how would you make it work?