I’ve wondered this a long time. From what I understand, it prevents cells from utilising oxygen. But I just watched a History show about the final days in Hitler’s bunker, and they describe biting down on the capsule and immediately convulsing for a moment, then death. How does a capsule work so quickly?
The only ones who know are dead 
Some of its contents are inhaled, following which cyanide is present near immediately in the blood going to the brain (and every else in the body). The cyanide reaching the brain causes it to cease functioning virtually instantaneously (by shutting down its energy production apparatus).
If KarlGauss is right about the mechanism of death being primarily inhaled cyanide fumes, and if this site is correct about how long it takes inhaled nicotine to reach the brain, then I don’t see any reason for it to take longer than 7 seconds for the cyanide to kill someone.
As you said, the way cyanide works is it interferes with the ‘Kreb cycle’ in cells; cells necessarily use oxygen to respire, and cyanide inhibits them from being able to use the oxygen. Because the Kreb cycle happens quickly and continuously, if the oxygen flow is interrupted, the cell can die after only a few cycles. If the cyanide has been inhaled it’ll reach the brain and start to wipe out cells en masse within seconds.
Although having said that, it seems like it’s not quite as quick as you’re thinking: by the sounds of things someone who’s inhaled cyanide may lose consciousness within 30 seconds, but it’ll still take them a few minutes at least to die.
Or to say it with fewer words: Cyanide stops cellular respiration and thereby blocks aerobic energy production in these cells.
I get that.
I don’t understand how biting on the capsule allows it to get to all those cells so quickly. In the show I watched, they tested a capsule on hitlers dog. The eyewitness said they placed the capsule in the dogs mouth, pressed on the dogs jaws to crush the capsule and the dog immediately collapsed.
Even if it doesn’t technically cause *death * immediately, it appears to at least cause unconsciousness. I would think that something orally ingested would take a few minutes at least to have an effect.
Things in your mouth can get absorbed into the bloodstream surprisingly quickly, even without being digested. Cyanide salts are pretty small molecules and may be absorbed directly in some configurations.
As for why it works so quickly, remember that it doesn’t have to go around to all your cells and screw them up. Once it gets into your brain and blocks its ATP production, your whole body is immediately kaput. In any case, it’s never instant, it takes ten seconds or so for inhaled hydrogen cyanide gas to be absorbed and bork your brain. Dunno about ingested varieties.
“Ingested orally” in the case of HCN doesn’t mean “absorbed through your intestines;” HCN is a gas (its salts like KCN and NaCN form some HCN in contact with water or saliva; Prussian blue isn’t exactly a salt, in it the CN is forming a complex with stronger bonds than in the KCN and NaCN) and it can be absorbed through the lungs, as KarlGauss mentioned.
And it’s Krebs’ cycle, please 
Might there be a big difference between gas and oral delivery of HCN? It takes several minutes for a condemned person to die in a gas chamber with some particularly drawn out cases.
Concentration is the big difference.
In the case of the oral delivery, you’re taking a more concentrated amount than what’s in the chamber at first, it takes a while for the cyanide concentration to build up in the chamber.
You can absorb somethings straight through your gums/mouth. That’s why people chew tobacco.
Also in the gas chamber when they refer to people ‘taking a few minutes (or more) to die’, I suspect in most of those cases the prisoner’s probably not conscious - but if they’re spasming and seizing, foaming at the mouth and making involuntary noises for a couple minutes it would look pretty horrible.
Might the classical “instant” cyanide death capsules instead be glass ampules containing the more water-reactive compounds? Lacerating the mouth should (might?) provide the soluble cyanide gas ready access to the bloodstream and brain. Inhalation would provide another means of contamination.
Maybe a tiny vial of acid embedded to speed to process along.
Just wagging.
I should add the obvious note that when talking about stuff like this, time to unconsciousness is all that really matters.
I’m not a doctor. At least not a medical doctor. But I can tell you all one thing. In the end, everyone dies of the exact same thing. Lack of blood flow to the brain. Regardless of the mechanism precipitating death, the process is still the same. If a man is shot through the brain stem, it shuts off all communication between the body and the brain. It appears as if the man dies immediately. He does not. Again, regardless of the mechanism death, it takes between 3-4 min. For the human brain to die due to lack of blood flow to the brain. In short, the answer to your question, with very few obvious exceptions, like being vaporized by a nuclear explosion, is no. You will not die immediately. Even if the blood flow to your brain is stopped immediately, it takes between 3-4 min. For all brain activity to cease.
I had the original Trivial Pursuit, too!
In the particular case of Nazis committing suicide that the OP’s in reference to, I’ve always read that glass ampoules of prussic acid (hydrogen cyanide) were exactly what they used.
Which is much faster than from 2009 to 2013, but maybe it takes longer with zombies. 
My grandmother had one and gave one, I think, to my Mom when they were fleeing the Nazis, in case they were arrested.
I’ll ask my Mom. (She has Alzheimer’s so I might come up empty). My grandmother’s war-time diary is in Yad Vashem in Israel, so that’s out for now.
On its use in the last days of the Third Reich, according to Speer one of the points of conversation in the Fuhrerbunker was which was quicker - shooting yourself in the head or the cyanide capsule.
Hitler himself was extremely suspicious of the capsules, an SS favourite, after Himmler’s betrayal with his attempted negotiation with the western allies through the Swiss. Hitler was paranoid about someone in the bunker rendering him unconscious and delivering him to the Soviets in exchange for clemency. Incidentally Himmler also dodged the gallows with cyanide (as did Goering - with potassium cyanide, as opposed to hydrogen cyanide used by the Hitlers), the wiki on him states; “The doctor attempted to examine the inside of Himmler’s mouth, but the prisoner was reluctant to open it and jerked his head away. Himmler then bit into a hidden cyanide pill and collapsed onto the floor. He was dead within fifteen minutes.[176]”
As a result, he gave one to his dog, Blondi, who died instantly. After that he was satisfied, evidence suggests he bit down on a capsule and simultaneously shot himself in the temple, while Eva just took the cyanide. Not long before the Goebels had forced their children to take cyanide - evidence suggest that the eldest struggled against her fate.