For less page flipping, I will quote the relevant sections from the rulebook

Stun (Full Action) [p. 191]

When unarmed or armed with a melee weapon, you can strike to Stun rather than attempt to land a killing blow. Whilst any character may attempt this action, only characters with the Takedown talent stand a good chance of succeeding. Striking to Stun is resoved in a similar way to making a standard attack. Test Weapon Skill. If you don't have Takedown, take a -20 to the test. On a success, roll 1D10 and add your Strength Bonus. Your opponent rolls 1D10 and adds his toughness bonus +1 per Armour Point protecting his head. If your roll is equal to or higher,your opponent is Stunned for a number of rounds equal to the difference between the rolls and gains one level of Fatigue.

If I understand it correctly, this Attack is insanely useful when unarmed, as the Attack automatically targets the head (normally the least armoured area of your enemy), if there is Armour it doesn't count double as usual, and you also have a comparably high chance of dealing fatigue damage (and of course Stun) compared to a normal Unarmed Attack that has a very slim chance of actually damaging of even fatiguing anyone who is only slightly armoured.

Takedown (Talent) [p.122]

As a Half Action you may declare that you are attempting to Takedown an opponent before testing Weapon Skill. If you hit and do at least one point of Damage, this damage is ignored and your opponent must make a Toughness test or be stunned for one round.

First thing: This does in no way mention the removal of the Malus to Stun Actions. If it's not too late FFG, please add that to the next version of the Dark Heresy Core Rulebook.

As I understand it, the Takedown Talent gives you a Bonus on the Stun Attack as well as a new Action called Takedown that works differently.

Because several parts of the rulebook were a bit confusing (the Becoming Stunned section [p. 200] not mentioning the Stun Attack for example), I just wanted to make sure I understood this correctly.

If I do, I'm definitely looking forward to buy Takedown for my Feral World Guardsman, who can then proceed to Stun ► Grapple ► Fatigue KO every heavily armoured/armed Enemy he comes across while nekkid.

My GM just decided to take out the whole Fatigue thing, so I guess it's not important anymore. Can't say I like the fact I won't be KOing Plaguebearers with bare hands now, but I got to admit it's a tad overpowered.

First let me say I'm not a fan of the unarmed combat set up in DH. But you did miss some important things with your assessment of the Stun maneuver when attacking unarmed.

1. If you are using the Stun attack unarmed vs. an opponent wearing modern (non-primitive) armour AP's are doubled
2. The attack does not (by the description) auto-locate the head. (there are a lot of ways to stun someone; just ask Harry Houdini)
3. Lastly if the target of the attack is armed the attacker takes a -20 on the attack (-40 without takedown)

I think the OP is arguing that the Stun mechanic actually ignores your first and second points, Guardianforever.

He's saying that since the description provides a different mechanic than damage (that of opposed 1d10 rolls with specified bonuses), the normal rules for doubling armor points against primitive and unarmed attacks doesn't apply. (Note, I disagree with this, even RAW - I'd say that the previous text of "resolved in a similar way to making a Standard Attack" means all the normal mechanics associated with unarmed and melee attacks, including the doubling of Armor Points against hits from primitive weapons, still apply... Stun just replaces the Damage and Penetration.).

That same mechanic also specifies that the opponent ALWAYS uses the Armor Points that protect his head, seemingly regardless of where your attack actually hits.

Your third point is absolutely true, but the OP's character could pretty easily be ALMOST "nekkid" with some brass knuckles, negating that -20 penalty on unarmed strikes.

Brass knuckles are no longer "unarmed".

So say Meathead MacBaggus decides to show off his feral world manliness and strips down to nothing but a loincloth and wades in to stun all his foes. The first guy he meets is "Nails" the hiveganger. Nails steers clear of this weirdo (he is creepy and clearly has no wallet to steal anyways) with the 41st millenium equivalent of "******, you gay!" [Yes, I spelled it this way to make sure no one takes it to be racial/sexual/whatever slander. It is a recurring joke in the TV show "the Boondocks"]. The second guy he meets is Trooper Jones of the Scintilla PDF. Jones is a little leery of the situation, but will probably stand his ground with faith in the Emperor and his body armour. The third guy he meets is Kasrkin-Sergeant Sturm of the 188th Cadian. Sturm sneers at the naked primitive and casually diembowels him with a nasty combat knife.

The moral: Pick on the lightly or unarmoured opponents with this tactic unless you are especially well trained and equipped for it. The big cookie you get from Takedown is that inflicting a single point of "damage" (yes, you still roll to be sure you overcome TB and AP) inflicts an unmodified toughness roll on the target. The "Stun" action grants a defensive bonus based on the AP worn on the head. So with Stun you desperately slam something reasonably hard against their helmet and hope it works.... With Takedown you flip up their visor and punch them in the nose, aim for pressure points or some other TRAINED method.