To accomplish a successful inertia opening is fairly simple.
DISCLAIMER #1 -- In today's market of high quality one-handed folders, the line between a gravity knife and a simple pocket knife is very thin. Some people will tell you that inertially opening your folder makes it a gravity knife and therefore illegal to carry in many states. I don't agree with this logic but it is in your best interest to consult you local statutes.
DISCLAIMER #2 -- Many people will tell you that inertial openings are abusive to folding knives. I consider this to be true some of the time. In my experience, a well built folder will take inertial openings fine. Still, there are some folders that will show significant wear from inertial openings. This wear is most apparent in some liner locks. I wore the liner lock on my AFCK extremely quickly through inertial openings, but the liner lock on my Military has stood up fine. It's very knife specific but be aware that some manufacturers might consider this to be abusive behavior and you might damage your knife.
DISCLAIMER #3 (thanks to calyth) -- Using knives is an inherently dangerous activity. Subjecting them to inertial forces while trying to hold on is even more dangerous. There is a chance that you could accidentally throw your knife trying this stuff. Please be careful! Don't whip your knife so hard you're about to lose hold of it. If possible, use a training knife with a dull blade or fasten a lanyard to the knife you want to practice with. Doing one of these things will make this a lot safer.
With all that crap out of the way, let me offer to you what I've learned over time. These openings utilize gross motor skills, rather than fine ones, and I think they are a valuable skill to have in your defensive arsenal. Not only are these defensive techniques, however. There are plenty of other stressful situations where a regular opening might prove difficult. What if you are working in such extreme cold that you can't feel the tips of your fingers? What if you are wearing thick gloves? At times like these, an inertial opening might be your best option.
Now let's evaluate basic techniques:
When most people attempt an inertia opening of a folding knife they will lock their forearm and flick their wrist outward.
THIS IS INCORRECT!!! This uses the knife's butt end as the rotation point but you want to rotate around the pivot of the knife. If you think about it, rotating around the pivot makes perfect sense, but every time I see someone try to flick a knife out, they do it in the manner shown above.
The correct way to perform an inertia opening is to rotate your wrist around the pivot of the knife. Try pinching the pivot between your non-dominant index finger and thumb. Then apply a normal grip to the knife with your dominant hand and rotate the entire knife while pinching with your other hand. This should give you an idea of the motion you want to accomplish.
In the following pictures, positioning of the fingers does not
matter. The important factors are the wrist, the forearm, and the proper arc of
Your first option looks like this:
You start with your wrist in an upward cocked position. Rotate your wrist and forearm down in a quick snap while remembering to pivot around the knife's natural pivot point. If you do this correctly the blade should swing upwards and lock open. From here you can easily transition into any forward grip:
An easier option:
This is an easier option because you are using gravity to your advantage. Just reverse the motion shown above so that your arc is moving downward. The wrist flip on this one is easy and the blade will fly out with authority. From here any reverse grip can be utilized.
Another opening style which utilizes gravity is the "Spydie-Drop." This is obviously easiest with Spyderco knives or other knives with large amounts of exposed blade steel while closed:
Just grip your Spydie's thumbhole between your thumb and index finger and jerk the handle downward with your wrist. This will swing the blade's weight downward and lock it into place. This is another easy opening but you should be extremely careful when utilizing this technique! You should note that you do not have a very sure grip on the knife when you perform this opening and it is very easy to send your knife rocketing out of your hand straight at the ground (or your foot!) I wouldn't recommend this over the other techniques, but it is probably the quickest way to draw a tip-down knife with a thumb-hole.
Finally, there is a more unorthodox opening:
This is a simple opening but it is not very fluid. You acquire your normal grip, then flip the knife downward, then flip it back up. This makes for a very jerky motion and can affect your grip on the knife. I don't really suggest using this method but it is more efficient in tighter spaces than the other openings.
A major mistake that people make when trying to do these openings is loosening the pivot of their folder to a ridiculous level. This is not necessary. Once you acquire the proper technique and work it into your muscle memory, you will be able to inertially open almost any folder from the factory.
The amount of force needed will vary between knives. As you practice with different models, you'll get a feel for what's necessary.
If you feel like I should add anything to this article, please