So, as it says, what is one really like? I've heard stupid (IMO) stuff that describes it as being anchored into your cranium, covered in crystals, and such, but I've seen SM Librarians wearing them, and they are often shown as part of the armor. Space Marines have times where they don't sport their armor, so how would that work? I'm a Psyker whose powers have great reach. Why can't it just rest on my head, and react to my will?

How attached to one is my character, Primaris or Librarian? In my grand equip scheme for a high-ranking Primaris, the hood is on the list, along with the other "cheesy" things. In this scenario, the Primaris is with a separate group that got their Vendetta shot down, and he was unconscious when captured. The Dom forces sequestered him in one of their city's former Black Boxes, where they once held Psykers for the arrivals of the Black Ships. The group of players have to retrieve him, if possible, but he was stripped of his gear before he woke up, and it is someplace else. Some of it is write off able (his carapace armor, flak coat, and plasma pistol), but his Force weapon, hood, and refractor are a bit more cool, and he's on-loan to their Command staff from an Inquisitor, so he can tell the group "we need to steal back my things". If captured, could they have removed the Hood, to put it in storage? Would it have broken in the act, or killed him? In a Psyker room, he can't channel, and thus it's sort of a moot point (the hood) anyway, but if they wanted to confiscate all his stuff, it would likely be among them.

I imagine the removal of a psyhic hood (especiialy under duress or by untrained captors) would be a gruelling process for the psyker in question as the device interfaces with their brain directly and one overly rough wire pull could trigger all kinds of trauma.

That said it`s a crystalline array so there`s not likely to be any moving parts and considering that the object itself isn`t lumbered with a pile of armour (even space marine variants expose the crystals) one must assume they are considerably durable.So it probably wouldn`t be roken unless they put some effort into it.

If you want to use a twisted cruelty factor (I would) you could have the dominate guys pervert (or hex-ward) the hood to emit unbearable feedback into the primaris`s mind rendering him practically unconsious and half-mad with agony rather than imprisoning him in a black box.

The evrans assume this renders hims safe... but it doesn`t it just makes him lose focus (not good.)

That way your PCs have to decide if it`s better to drag an inquisitor`s delirious pet psyker back to extraction and hope he doesn`t get the all killed or try to remove his hood without any training and risk killing him, themselves and possibly triggering a massive psychic incident.

You can have random psychic phenomena sparking off every few minutes, psychic deus ex machina and occaisional flashes of strained sanity as the primaris stuggles to keep in control despite the pain.

After all what`s more grimdark than psykers in pain lashing out uncontrollably and endangering all nearby? A friendly psyker you can`t just shoot in the head doing it.

The fluff clearly states that the hood is attached to the cranium of the psyker... Most of the device is outside the head but it is "anchored" through the skull of the psyker.

So having it removed is presumably a delicate process. Then again, if the enemies weren't that concerned over the wellbeing of their prisoners they might have broken off some of the bits or otherwise damaged it while removing it.

OR... Maybe it is like the connectors that the Matrix characters have and there are "sockets" on the skull of the psyker where the hood connects making the attach/remove process safe and quick. Even so much that the psyker did not suffer permanent brain damage. This actually seems likely, as Space marines for example wear hoods that look like they are part of the armour so it would make sense for the hood to have such a way for quick attachment. Rather than performing brain surgery before and after every battle. ( "Ooops, the ship bounced a bit and I accidentally Tigurius's brain..." )

Psychic Hoods are plugged into cranial ports akin to USB ports, and we all know how easy those are to unplug.