I'm interested in the mechanics of heroically throwing yourself in front of a shooting attack against an ally. (As distinct from, say, a prolonged period of actively defending an ally from multiple attacks.)

In the Fate Core section on resolving attacks, there's [a note](http://fate-srd.com/fate-core/conflicts#resolving-attacks):

> 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.

I don't understand how this takes place in the turn order of conflict. FC doesn't have the concept of delayed or triggered actions, so how can a character interrupt an attack made against an ally? On their turn before the attack, the attack won't have been made; on their turn after, it will already have been resolved. If they just move on their turn so that they're shielding their ally, why wouldn't their defence just be as per normal?

What would a turn-by-turn account of this kind of action look like?