Getting "taken out" in Fate seems relatively straightforward, at first. From p.168 of Fate Core:
If you don’t have any stress or consequences left to buy off all the shifts of a hit, that means you’re taken out.
Additionally, from this website (emphasis mine):
When taking stress, the box with a number matching the amount of stress taken is checked off. If that box is already checked off, the next higher unchecked box is checked. If no available higher boxes exist, the character is taken out — removed from the conflict, or at least from the dimension of the conflict represented by the stress track.
But can a character, after taking a hit, decide to be taken out? Not a concession; all the "bad stuff" from being taken out results. For example:
Jenny is in relatively good shape, having not taken much stress or consequences in the conflict, so far. Thus, naturally, when an NPC swings at her, she doesn't choose to concede the conflict before the roll.
Sadly, after an abysmal defense roll and a phenomenal attack by the NPC, Jenny get whalloped with a hit that would require her to take consequences (maybe even two or more)!
Now, Jenny could, theoretically, stay in the fight and take consequences and/or stress boxes. She also knows it's far too late to concede and get a nicer situation and tasty Fate points. Could she instead succumb to the hit and allow herself to be taken out, instead?
Best Answer
So far, we have two answers, which appear to contradict each other. I tend to think in practice they're not that far apart from each other, tho.
When you're taken out, you cede control over your fate to the attacker. That means the attacker can assert all sorts of things about what happens to you. Like: you're dead. And because total destruction of the character is on the table, so is any amount of change to the character. Like: you live, with all your consequence slots filled and needing healing. Or: I just knock you out, you'll have a bruise and maybe a headache for the next scene or so — let's call that a mild consequence (or not even that).
(And that's just assuming a physical conflict. In other contexts taken out might be, "And so I utterly change your view of your father. Change the aspect representing your relationship with him on your sheet to reflect the resulting estrangement.")
At any rate, all that nastiness that could befall you (consequences, etc) is still in play after you're taken out. So any notion of saying "sure, I'm taken out" as a way of avoiding consequences is bunk; the only way to avoid consequences for sure is to concede when it's properly time to concede.
So: how much does it matter if the dice hit the table, you take some stress and consequences, and then immediately concede (as has been said) vs giving up and having stuff at least as bad happen to you anyway? Not too much, IMO.
But, anyway, let's look at page 140: It's a summary of the effect, so you can quickly understand what success on an attack means; it's not procedure. There's a page reference to stress and consequences in the sidebar, thereby implying that the official procedure is found elsewhere: page 160.
Doppelgreener's on the mark by taking this text to heart: "If you get hit by an attack, one of two things happen: either you absorb the hit and stay in the fight, or you’re taken out."
As you read on through page 160-162, stress and consequences are referred to as options. Options imply choice. For me, that makes the procedure (essentially): look at value of hit; choose whether or not to take consequences to reduce it; look at post-consequence value; mark off appropriate stress box; if you can't (or won't, tho given that stress is super-ephemeral that'd be a weird line to draw in the sand) mark off the appropriate stress box, you're taken out.
What's being "forced", really, is the handling of the hit, with concession unavailable until after you handle it. Dice are on the table, that hit's coming your way, and you've gotta parse it out with your options or drop, and if you drop, you have no control over what happens to you. Choose to take consequences, and you'll have some control over what's happening to you (including, to a reasonable extent, how those consequences are described); choose not to take consequences and leave yourself with more stress than you can actually take, and you give up all control as you're taken out... including control over whether or not you end up with consequences.
As such, yeah, you could choose not to reduce an incoming hit with consequences (that's the real crux of all of this) and just get taken out by the resulting massive pile of stress. Because it doesn't really matter that you chose not to take those consequences. When you're taken out anything can be done to you: "the person who took you out gets to decide what your loss looks like and what happens to you after the conflict", page 168. I can't see how there's any value in forcing someone to retain control over their character when they don't want it, which is what choosing to be taken out (instead of conceding) is.
Apologies if this seems confused or rambly — it's late!