I often witness fighters in UFC survive direct blows to their nose. Punches with such extreme force can knock a person unconscious.
Even soccer players are knocked unconscious if the Ball hits them on their face with force.
So, how do fighters (MMA, Boxers etc) survive such punches to their nose?
As others have said, a knockout is typically not the result of a blow to the nose, but to the chin.
The brain is basically a loose spongy thing trapped inside your skull. When you receive a hard blow to the head, the brain will hit the inside of your skull – much like when you shake a nut, and you can hear it rattles against the shell. This will temporarilly shut down the brain, causing a trauma otherwise known as a "knockout".
When you hit the front of the face, the head is moving rapidly in one direction. When you hit the jaw from an angle, the head is not only moving backwards, but it also rotates, thus making the brain more likely to bounce against the inside of your skull harder or more times – thus making the trauma more likely or severe.
To answer your question, there are several ways a fighter can prevent this. The most predominant precaution, is to train the neck muscles. The stronger the neck, the harder blows you can sustain. Fighters will also warm up the neck muscles before a fight.
Secondly, it is all about practicing for head blows, getting hit a few times in a controlled environment, and know how to handle it. If you are constantly sparring or doing drills with head blows, you are more likely to react. Ideally you would block, parry or evade, but the last method of defense is to "brace for impact".
That said, a well placed kick or strike to the tip of the jaw is baically like hitting an off-switch. It doesn't even have to be very powerful, it just requires a light tap. I've been on the delivering end of it a few times, and I am always surprised as to how little is actually nescesarry for a knockout. Ironically, it has never been my intention to knock people out with the technique, it has always been intended as a setup for the REAL knockout technique.
Likely effects of a hit to the nose, is a broken nose. the nose is basically cartilage (not bones), so it's fairly easy to crush. It will bleed profusely, and in some cases you will not be able to continue the fight, if they cannot stop or control the bleeding. You might suffer cosmetic injuries, but for the most part a broken nose is an easy fix.
Likely effect of a good hit on the chin, is immediate knockout. If the brain trauma is severe, you might suffer from light or severe memory loss. The jaw might dislocate or break, and you might loose some teeth in the process (even if using a mouth guard). Cosmetic injuries might happen, and you might need surgery which may or may not be invasive, and may or may not have a long term effect. Some organizations may bar you from sparring or fighting for a temporary period.
Fortunatelly, at most tournaments where full contact techniques to the face is allowed, the fighters are also well prepared with years of practice. They know how to prevent getting hit, and they probably already have several fights under their belt under a bit softer ruleset. They know the risk – and often have to sign documents that states exactly this.
This is really simple. Not every impact to the head (whether jaw or nose) knocks someone unconscious. Plenty of soccer players get hit hard by the ball in the face and don't get knocked out.
You're more likely to be knocked out if you're weak, if you don't see the impact coming, or if you have a history of being knocked out (that is, people can develop glass chins). Being strong and practicing good defense like head movement and punch awareness helps fighters avoid the knockout even when they're hit hard.
A blow to the nose is no more likely to knock you out than one to anywhere else on the face and head so I'm not sure why you've chosen the nose. There is a much higher chance of knock out by a sideways blow to the chin or straight to base of the skull or temple.
A knock out is basically sudden trauma your brain can't deal with. A sideways blow to the chin causes a rapid and powerful shake of the head which is very likely to cause a knock out and it's why boxers are encouraged to hold their gloves to their chin. The two other methods are soft spots in your skull and there is less bone mass ergo less power is needed to cause trauma. It's against the rules in boxing to punch the base of the skull and the temple is generally protected by the gloves as it's a small area and force is spread over the area of the glove.
Granted enough power can cause trauma anywhere on your skull but a blow to the nose -like I said- is no more likely than anywhere else.
If you watch a number of knockouts, you'll see that in each case, the guy's head moves when he gets knocked out. On the other hand, your MMA fighters' heads don't move when they get punched. It's the sudden movement that slams the brain into the skull that causes the knockout. Think of a car running into a wall. If it goes through, the people inside are fine. If the wall stops it, the people inside come flying out.
Along with the comments above about how knockouts actually happen, it's worth mentioning also that losing consciousness does not necessarily involve fainting. As the name might indicate, it's about conscious movement, and the repeated training means a fighter may continue acting for several seconds, sometimes enough to recover. It's actually a danger in the ring that referees have to look out for, when a fighter is literally going through the motions, because it increases the risk of them harming themselves, and because further head trauma at that point can lead to severe injuries.
