There is a story, one of many, about the creation of the Bowie knife.
We are told the Jim Bowie's brother Rezin was out on his property one day when he was attacked by a wild cow. It the ensuing ruckus, Rezin managed to stab the cow to death with his belt knife.
unfortunately, during one or more of his thrusts, Rezin's knife slipped up his hand, lacerating his fingers quite badly.
As the tale goes, Rezin designed the Bowie knife, with its wide cross guard, while recovering from his wounds.
I don't know if the story is true, but it is quite "near-fetched" in that the sort of injury described is fairly common with people who try to stab someone or something with a knife.
This story illustrates one of the very important but sometimes overlooked aspects of knife work, grip training.
When a knife makes hard contact with an object you have to deal with all the ramifications of Newton's laws of motion. You also have to contend with the physics of a wedge moving through inconsistent material. If you have not learned to deal with these factors you may either lose your knife or injure yourself.
The first area to work in is grip strength and finger articulation.
You will want your grip to be both strong and supple because you will be changing pressure on your knife handle constantly.
There are two things I do to keep my grip strong, the first is that I keep a Gel ball near at hand and use it throughout the day. There is a nice side effect to this I am told, in that using a gel ball is supposed to help prevent things like carpal tunnel.
I prefer a gel ball over devices with springs and the like because you have more options when it comes to using it. You can work one finger at a time and squeeze at odd angles.
You will also want to learn a few mobility and coordination exercises for your fingers. One good way to do this is to get a book on basic "slight of hand" magic and practice the exercises found in it. You can also benefit from the wrist and finger exercises in the warrior wellness program.
The other thing I do to develop grip strength and articulation is to work out regularly with Clubbells. The interesting thing about these training tools is that your grip strength is the determining factor on your ability to do the exercises. What this means is that no matter what other part of your body you are working, you will be dynamically working your grip.
When you actually make impact on something, whether with a cut or thrust, you will begin to appreciate how important it is to have a grip that is "alive" on the knife.
Unless a person practices Filipino martial arts, there is a better than small possibility that they have never dealt with weapon impact. But even for an arnis or eskrima player, stick on stick (or even body) impact is still substantially different than knife impact when it is cutting or stabbing something.
Of course there are problems with getting the necessary experience, after all you can't just grab live blades and start cutting each other up.
There is one way to get around this problem. The Japanese sword arts have a practice called Tameshigiri, which is usually translated as "test cutting". Western sword practice had the same sort of practice, but until recently we have not heard much about it.
Part of test cutting was to determine the quality of a given blade, but it was also done so that a swordsman could have some real experience in moving his sword through material that approximated flesh. You will find some video footage of Test cutting done in the western style here so that you can have an example of what I am talking about.
There has been substantially less work done on the subject of test cutting with a knife. I know of only one article on the subject, and while the author gives some good advice, his basic premise is flawed.
If you take a look at the three photographs at the top of the article you will see that the cut is started with the knife cocked at the ear and it finishes at the opposite hip. This means that the cut was a full swing that transverses the entire range of motion diagonally for the arm.
Most people, when fighting with a knife, especially a smaller one, for example a Gerber Profile
with a four inch blade, are not going to be making full arm swings with it (at least not if they want to live).
Here is an important truth. The way you train will be the way you fight.
So if in training you punch short, or off to one side, when you really need to use your skills you are likely to do the same thing, and if you practice your test cutting with a full arm swing you might try to use the same technique when it counts.
In my opinion, a person should train as realistically as possible, and since it is impractical to cut up your training partners you need to be "practice cutting" on various materials using techniques that you would actually call upon in a real fight.
What you will want is to be able to cut material that will allow you to experience things like cutting along a grain, which will try to turn the knife in your hand, or thrusting into a material that will give you the chance to learn how to not let your hand slip up the knife blade even if you hit something that approximates bone.
(Note: when I start training someone in thrusting skills I begin with a knife that has a dulled blade but a sharp point. This is because their hand will slip at some point in their training and without this bit of protection they will sustain severe injury).
(Another Note: If you choose to practice this sort of training you do so at your own risk. It can be very dangerous and you run the risk of hurting yourself or someone else quit badly. You assume all risk for trying out these things).
The materials I use for practice cutting are newspaper, cardboard, office carpet, closed cell packing foam (like a computer comes packed in, not Styrofoam) paper towel tubes, Cardboard tubes that carpet comes rolled on, straw mat (tatami) in various thickness, bamboo, various items of worn out clothing such as heavy jackets, 1 x 10 pine board. I will also use the occasional plastic pop bottle.
I use thick wall PVC pipe and 2 x 4's to build stands, and I use a lot of heavy string and Duct tape.
What you will want to do is create exercises that simulate how you will use a knife in combat (that is to say not full arm swings) that will allow you to see just how well you actually cut or pierce training material.
For example take a 3/4 inch PVC pipe and wrap and tape it with newspaper to about the thickness of a forearm, then cover that with the sleeve of a heavy jacket. Mount this so that is is at the same angle and height as the arm of an opponent. Discover if you can cut through the jacket and into the newspaper using a realistic cut.
One thing you will quickly discover is that the only parts of a knife, especially a small knife that count are the edge and point.
If you hit someone with the side of a stick, it is no big deal, it still does damage. If you hit someone with the side of a four inch blade, the best you can hope for is to piss them off.
To cut most effectively the edge of your knife should be at 90 degrees to the material being cut. For every degree your edge is more acute or obtuse, you will run the risk of the material you cut turning the knife because of "grain". To the degree your edge is more acute or obtuse that the direction of force, you lose cutting power.
Only by actually cutting something can you learn at the "body" level how to deliver the cut effectively and how to respond to the Newtonian consequences of delivering the cut.
Here is another consideration for you. If you are using a folding knife, it is very likely that at some point the lock will fail on you. This is especially true with cheap folders, but is also true for the better ones.
"But Wait" you say. "I have one of those knives that have good locks!" I have a picture of it in a vice with a hundred pound weight hanging from the handle!"
Well, that is groovy and all. I am sure that it is difficult to make the lock fail in the "closed" direction, but in cutting, the force is applied to the blade in the "open" direction. That is to say exactly the opposite of the test pictures that knife manufacturers show you. You will also want to notice that the force applied to the blade in such tests is not "impact" force.
I have yet to find a folding knife that won't fail towards the "open" direction after a time, this does not mean that there is not one out there, just that I haven't found it. I have however found this to be particularly true for knives with all plastic handles, even the best plastic seems to crack under repeated impact.
When practicing thrusts I like to use material that "grabs" the blade. Some closed cell packing foams are really good for this. I set up a half inch layer of newspaper over about two inches of such foam. The knife will tend to try to stop on entry, so you get to practice not running your hand up the blade, and will tend to "grab" the blade so that withdrawal is not easy. (remember train with a dulled edge for safety) Putting a piece of pine board as a backing will simulate hitting bone.
Performing a perfect cut with a knife is a very "zen" sort of act. It requires that your structure, breath, and motion be in complete harmony. This becomes even more evident when you use good fighting technique. It is impressive to cut through a two inch roll of newspaper hung from a string with a 12 inch bowie knife and a full arm swing, but it is even more telling if you do it with a 4 inch blade and a cut that draws you back into a guard position.
Have fun but be safe.