Hello there, I`ve thinkin' for a while on this. When usin' a knife as a weapon people tend to use it this way...
Example image 1
This way, with the blade above the thumb. I think this way is kind of uncomfy, but also less effective. Why? We can say that all movements of our upper limbs goes straight to our torso, or at least, they works better on that diredtion.
At this point the arm's movements must be suited exclusively for the blade. Any other attemp to move the limb may be uncomfortable or dangerous. So I started to think, why can't be a knife better used this way...
Example image 2
With the blade below the thumb, this "Mantis handle". I think in melee, this position gives you a better performance of both your weapon, and body movements. Let's say you are tryin' to assest a series punchin's. You may either hit your foe in the side of it's head. Or seriously injure it with the blade. Also, your blade won't become a risk of injury for you if you ever miss a hit or lose balance.
Why? Well, in my point of view, the blade is always lookin' out of the body due to the wrist's natural stance. In addition, I think the hand grip (sorry, got problems here with the translation) gets stronger. Since any force exerted to the blade won't fall over the thumb, but directly in the palm. Which, IMO, works for a backin' instead of a point of pressure, like the thumb should be.
So, you can both hit with your fist, slash with your knife at tthe same time, then use a stronger block with the blade in case of your foe havin' another weapon (I think if you get enough close to a guy armed with a gun, you can move the gun away from you with the blade).
that's what I think about it, since many times I've seen soldier trainin', the tend to use the knive with the blade above the thumb. Otherwise, the knife is to big to be weilded downsided (for example a machete).