I haven't ever done knife training, so I wouldn't know. I just thought I'd throw it up there since I saw it recently and thought it was interesting.
But, would you mind telling me what's wrong with the technique? I'm already not going to use it since you said there are so many things wrong with it, but I'm hungry for knowledge and learning what not to do is one of the best ways for me to understand why what you should do is correct.
Thanks!
Okay here are the main issues:
First off, I have just a general gripe. He is not holding the knife correctly in his attack. Perhaps the attacker will be completely untrained and hold the knife that way, but there are also people who practice using the knife which is why they carry it.
Notice how he holds the knife like a hammer; the point of the knife sticks straight up perpendicular to his forearm bone. This is one of the most awkward and useless ways to hold a knife. Try holding a pen this way and see how easily you can cut with it. It's really tough to actually control where it goes and do what you want.
Now hold the knife properly by aligning the point parallel to your forearm bone. You basically cradle the knife in your fingers. In this way, the knife becomes an extension of your own hand. You can now use your fingers to easily manipulate the point and move it much more easily. Also notice how much extra range you just gained with the knife. In the hammer grip, your range extends as far as your hand reaches. With the proper forward grip, your range extends the extra inches of the knife blade. Those extra inches count for a lot.
With that out of the way, my biggest objection is easily his footwork.
Notice how he steps into the knife attack and blocks it. This is a huge mistake because you can easily miss with your block. With the way he is stepping in, if that block fails, he will stabbed badly right in the stomach or chest. Just watch that realistic knife defense video earlier in the thread for an example of how easily a block like that can fail against a real knife attack.
The most important part when defending against any weapon is to use your footwork to get off line. You want to avoid the knife attack by moving away from it. If it is a straight on thrust, you want to circle around (conceptually like a bull fighter pass in BJJ) so that the thrust misses you completely. Then once it misses you, you can try to seize it from the side. For slashes, you again move away to protect yourself. When you move away from the strike, you give yourself more time to react since you are increasing the distance the weapon has to travel. When you move towards the strike, you give yourself less time to react since you are decreasing the distance.
Weapon attacks already happen incredibly quickly. You need as much reaction time as possible. You also need to protect yourself with your footwork as much as you can so that if you miss with your hands, you don't end up with a fatal wound. If you step into the knife attack like they show and hope that your block works, the odds are that it is going to fail anyway and you will be dead for your gamble.
A secondary point of concern is the method of seizing the weapon hand. If you notice from the video, they do not seize the hand. Instead they control the elbow to twist the arm.
You always want at least one hand of yours controlling the wrist. Ideally, you really want two. If you don't control the wrist, the knife can slash out in unpredictable directions and cut you. If you hold the knife in a stupid way like they are showing there, it is much harder for this to happen. However, try holding the knife the proper way I described. Now control your elbow with one hand and move the point of the knife around using just your wrist and lower arm with the other hand. Notice how much control you still have and how far you can reach and cut. Granted these are not particularly powerful strikes, but with a sharp knife it does not take very much force to cut into someone.
Those are my objections to the video as shown. They are critical mistakes that will get you killed against a real attack. If you want to prove it, have your training partner attack you with a rubber knife for real. Attempt to use that technique to defend yourself. After having your guts spilled onto the floor a few hundred times, hopefully you will realize that you can't step directly into the path of a real knife attack that way.