Knife defenses, and fighting with a knife in general, are voodoo sciences. It is very difficult to train, with any sense of realism, how to defend against a knife wielding attacker. The reason that Brazilian Jiu-Jitsu is such a practical, powerful and useful martial art is due to how realistically the techniques can be applied in training. Striking arts cannot be trained as realistically as grappling arts. Weapons arts have to be trained even less realistically.
I am NOT saying any of this to insinuate that training in weapons or weapons defenses is a waste of time. Just the opposite: we have to exercise extreme discretion in how we train against weapons. Because there are so many variables to a weapon attack that must be estimated in practice, some practitioners tend to take a bit too much license in making up ways to defend and disarm an attacker. I have seen some dangerously ignorant approaches taken in this regard. Many disarms you might see floating around martial arts schools or videos online might feature an “attacker” leaving his weapon hand floating in space while the defender makes an elaborate series of strikes before stripping the knife away.
As I say in the video, statistically (for what that’s worth) most people who will stab you won’t reveal the knife until it is inside you. Don’t watch movies or most martial arts techniques to learn anything about knife attacks, watch prison videos. Survivors of knife fights go to the hospital; losers to the morgue. I do my best to research and pressure test everything I ever show or teach, and knife stuff is the hardest even though I have access to some of the most brilliant minds on the subject in the world.
Without further ado, here is a video of what I feel to be a realistic defense in to a potentially realistic knife attack. I hope you enjoy.