Thank you all for the discussion, your enthusiasm for the Yo2, and your questions.
With regard to the utilitarian function of the Yo2--absolutely. I grew up in pretty humble surroundings. Although we didn't have much, my Dad was very smart and skilled with his hands. I learned that the best way to have stuff was to make it, so I used to spend hours making toys out of cardboard, scrap wood, string, and anything else I could scrounge. One of my most prized possessions back then was an X-Acto knife set my Dad bought me with all the different blade shapes. Initially, I thought the different blades were cool and spent time swapping blades to cut different materials and shapes. After a while, though, I realized that the standard straight cutting edge was the most versatile. If I needed precision, the tip did the job. At the same time, if I needed cutting power, the straight cutting edge transferred power all the way to the point.
The blade for the original Yojimbo was based very much on these experiences and the shape of a common utility knife/box cutter. It was designed before 9/11, but released after. Promoting it as a "box cutter on steroids" was not appropriate at that time, but functionally, that's exactly what it was. The Yo2 does the same thing, but even better.
With regard to the blade hole, I designed it to fit with the natural mechanics of the hand and what works best under stress. I also tried to keep the size of the handle comfortable while keeping the width of the closed knife as minimal as possible.
When most people hold a closed knife naturally in their hands, the back of the hand is at about a 45-degree angle to the floor and the fingers are curled around the clip, which raises the plane of the knife handle to about 45-degrees as well. This is a natural, comfortable position. Conversely, holding the knife so the closed handle is completely horizontal or completely vertical forces you to raise or lower your elbow or cant your wrist to unnatural angles.
The most efficient motion of the thumb--especially under stress--is a straight, linear drive. Moving your thumb in an arc is a complex motor skill--one that quickly goes away under stress. When the Yo2 is held as described above, and the thumb is placed naturally in the "scallop" in the handle, the ball of the thumb engages the functional surface of the hole--the area immediately adjacent to the steel marking on the ricasso. In that position, a straight drive toward the tip of the handle's lower guard provides excellent leverage to open the blade quickly and positively. In the remaining part of the arc of travel, the thumb goes along for the ride to open the blade fully. In doing so, it rides safely clear of the edge so it doesn't get "shaved" in the process (folks who insist on opening their folders by holding them with the plane of the handle vertically are prone to doing this).
Using this technique, the entire hole does not have to be exposed--just the functional part. That allows the handle to be slightly wider at the area of the scallop, which provides a more comfortable and secure grip. It also helps the handle to lie flat against the palm, making it easier to orient the plane of the blade during ballistic cutting, preventing the handle from twisting, and allowing greater torque when executing "comma cuts."
If you prefer placing your entire thumb on the blade hole and tracing an arc to open your knife, you won't like the Yo2. However, you may also be working harder than necessary to open your blade and, in doing so, my be developing an opening style that is inconsistent with high-stress applications.
Also, just to clarify, a "kinetic" opening uses a projection on the blade as a lever to open the knife--usually off an opponent's body. This was one of the defining features of the Gunting. An opening that relies on quickly rotating the closed knife, stopping the handle, and allowing the mass and inertia of the blade to carry it to the open position is an "inertial opening." Properly designed knives with solid detents or self-close mechanisms make this action one of deliberate and practiced skill. Knives that don't make it a challenge are often unsafe to carry.
I hope this helps. Thanks again for sharing my passion for the Yo2 design.
Spyderco Special Projects Coordinator
Founder and Lead Instructor, Martial Blade Concepts