Over many years of martial arts training, I have seen some horrible techniques in knife attack defense. I've spotted well respected instructors teaching and advocating techniques that looked good against a compliant partner, but in no way would work against a knife-wielding maniac. Just the other day, I heard two guys discussing how to take on an assailant with a knife. One was talking discussing how he could kick the knife out of the bad guy's hand, and the other said he could block the strike and then knock the attacker out. Similar conversations can be heard in and out of the self-defense community. There's more to these situations than meets the eye quite often, and it's good to listen to different perspectives on the subject.
Self-defense instructor Paul Vuncak makes a great point with his beef slash demonstration. He says that simply trying to take a knife on shouldn't be your first instinct. Matter of fact, you'll be cut and, most likely, very badly. All techniques must take this into account. In my opinion, there is only one way to train against knife attacks (it is understood in this scenario that I do not have a firearm).
Rule No. 1: RUN. Get as far away from the individual as possible. The good thing about knives is they have limited reach.
Rule No. 2: If you cannot run, put a stationary object between you and the attacker. Find a car, wall, large piece of furniture....something.
Rule No. 3: If you cannot run and there is nothing to put in between, pick something up and start swinging. Try to even up the odds, find a stick, chair or a blunt object of some kind to make him think twice about getting close.
Rule No. 4: If the previous rules are not applicable, then follow the principles of Clear, Close, Control and Capture. Clear all your vitals from the path of the blade. Close the distance by jamming yourself super tight against them, so the attacker cannot swing or stab with the blade again. Control the arm holding the knife as close to the hand as possible. Capture the weapon by any means necessary. That means biting, clawing , striking , spitting, hitting, smashing; whatever it takes to extract it away from the attacker.
As a good experiment, give a person a marker and tell them they can mark you up as much as possible before you take it away. See what happens. It's often a real eye opener. Have you considered how you would handle a knife attack without a gun? What are some crazy techniques you have seen?