Target areas of weakness and possible vital points, mostly arteries and nerves. Specifically if you sufficiently hit the carotid artery once your assailant will lose consciousness in around 3 seconds.
I have seen it happen, someone stabbed multiple time in the neck, including severing a carotid artery. The large stabbed individual proceeded to beat the crap out of the punk that did it, and then passed out about 20 seconds later. Others got involved. With blood showering the area. The attacked individual got prompt medical attention and a blood transfusion and were back to normal within a couple days with just some scars and stitches.
The amount of blood showering out under pressure at face level certainly means any disease that person had was shared with his attacker.
I also knew someone knocked out and put into a coma from a hit to the head with something as mundane as a full beer bottle.
Stabbing offensively is much easier than defensively. The offensive attacker knows they need to use their weapon before they are engaged in a fight and can deploy it accordingly.
However I assume you are not planning criminal actions.
If the other individual starts the fight, they are not going to let you easily do damage. Most violent criminals that make a life out of violence have more experience fighting than your average person.
(Sure there is the young punks and drunks who that does not apply to.)
If they are using a weapon they will have similar or greater reach. If they don't care about the law they are likely to have a bigger and better weapon whether a large knife, blunt weapon, or a gun.
If they are not armed, then you are using lethal force to face a simple battery. Which in most circumstances is itself a crime, and you are at least likely to be considered a mutual combatant and face criminal charges for any serious damage.
Mutual combatants don't get to claim self defense.
If you end up on the ground quickly, retrieving a folded knife and deploying it before it is countered is not so easy. That simple little split second process both alerts them and it can be difficult to flick open a knife, while holding it tight enough not to drop it in mid struggle, and when someone has grabbed or moved to contain that hand so it has minimal movement.
A fixed blade is much easier, but also illegal in much of the nation.
Then after all that movement to retrieve that weapon with one hand, leaving you less able to counter attacks being delivered, you are left with a weapon with only moderate potential.
So if you deploy the weapon before an attack, you are committing the felony of aggravated assault/assault with a deadly weapon.
If you pull a knife on a single similar sized attacker and they do not have a weapon, you are meeting a misdemeanor assault or battery with a felony attack in most jurisdictions.
So under most circumstances where using that weapon is legal for you, the other individual will be armed, or there is more than one person attacking you.
A small folder is a really bad weapon in those circumstances.
It has poor reach, is hard to deploy while grappling, and does minimal damage compared to most weapons you can be attacked with.
If you have any opportunity to leave or back down and choose not to, then you are very likely to be considered a mutual combatant, and self defense is not an allowed defense.
An actual fight between grown men who know how to fight or lived some street tough lives when the stakes are lethal is quite different from a schoolyard brawl between teenagers. Anything and everything which can be a weapon suddenly is a weapon. There is a lot more power behind even the same attacks, and most solid hits do significant damage.
If it's not bolted down, it gets thrown, or picked up and smashed into an opponent.
And that is if they didn't bring a weapon, a likely much better weapon not in compliance with the law.
I am glad many here are so confident and content with tiny folding knives.
If you were attacking unarmed people, or other people with tiny knives your confidence may be warranted.
If on the other hand you are using legal self defense, it will only be deployed when facing another weapon or multiple attackers.
Where I come from the type of people who are multiple attackers would then pull out weapons they are carrying themselves and some not armed may back up and grab things in the room or off the ground. Whether it is a piece of furniture with greater reach, or some bricks or large rocks on the street. Or random debris, like a bottle, pipe, or some discarded automobile component.
If it is a lone individual they already have their weapon out to justify your use of lethal force.
stomp to the neck (after they hit the deck)
Which in most courts of law will be murder or manslaughter. A prosecutor will argue they were already down and you then used excessive force against.
You seem to forget a person abiding by the law dealing with authorities after the fact has limitations the criminal who will flee does not.
You may feel they are still a threat and will get up and hurt you, but I guarantee the jury which is half house wives and soccer moms, and men who have never experienced violence will by holding you to different standards. Standards which include ceasing to attack a downed and injured opponent.
The prosecutor will tell them they are right, and that someone down on the ground is not an immediate threat, and hence it was not self defense.
I just put a tool in my hand and strike the same way as if I didn't have the tool. I don't dance around and "knife fight" or "stick fight".
Which is fine as long as you are willing to take some stabbings in return from the armed opponent with that strategy. Then it is just who does better damage, but both will take damage.
Because clearly the other person is armed if you are fighting one individual with a lethal weapon.
To be legal you may not use a weapon against an unarmed man unless you can voice to your average person in a jury box just why they were so much more dangerous than you and created a major disparity of force barehanded.
In most cases they will need to be armed or the court will find you overreacted.
You must flee or back down from any challenge or threat if given the opportunity, or you will be considered a mutual combatant even if the other person attacks first.
Meaning your pride has to go out the window, or your use of force is unlikely to be legal. Mutual combatants cannot claim self defense, and so any damage inflicted even against someone who attacks you first is criminal.
You cannot finish off a downed opponent, and so if they proceed to get right back up and attack you, you must then fight for your life once again and potentially lose. Because they just as easily could have stayed down and no longer posed any threat.
You may not flee the scene without suspicion after wards in most circumstances, so if their criminal buddy, friend, or family member you were unaware of then attacks you in retaliation, well that is one of the added dangers of remaining on scene.
LEO and others may prevent you from immediately treating minor wounds or washing the other guy's blood from your wounds and chemically treating them to reduce transmission of blood-borne disease. They want to gather evidence and are likely to detain you initially if your injuries do not warrant a trip to the hospital.