My first experience in learning defenses against knife attacks came from ITF tae kwon do, in the form of individual sets of techniques practiced as static drills very similar to hapkido's. You know the type: uke attacks with the knife from a set stance with an overhead ice pick grip/forward hammer grip lunge/backhand slash/take your pick... the defensive action to follow as stylized and choreographed as any dance performance.
Now don't get me wrong, the techniques I learned were not much different than those from aikido, jujutsu and those extrapolated from various kata as bunkai from karate. Yet at the time something was missing from the training to make them useful out of the dojo and practical on the streets where the blades are not made of plastic and people don't get hurt when someone moves too fast or too late.
In the years that followed I learned many different techniques to defend against the knife, both armed and unarmed. From trapping and stick disarms in Filipino arts, to wrist locks and throws in aikido and jujutsu, to strikes and sweeps in karate; defensive tactics used by professional bodyguards and bouncers...some work quite well for me, others do not but I study them and try to find ways they might correlate with and enhance other techniques I have learned. It is this study that has given me an interesting insight which sounds obvious on the surface, yet it is forgotten often: the success or failure of any given technique is highly dependent on small details that can be easily overlooked during high stress situations.
Most martial arts and RBSD systems have definite lesson plans, patterns used to teach students the skills they deem will allow them to survive and prevail against a deadly surprise attack, armed or unarmed. Two man drills, sets or defense scenarios are practiced until the gross movements are ingrained, then faster execution follows to build reactive speed. Nothing wrong with this method, it is used by almost all systems in some form or another because it works well at its purpose: basic reflexive skill development under progressively difficult and changing circumstances. The problem is that sometimes such changing circumstances are not readily visible or identified and the end result is technique that works wonderfully one way but can be quite dangerous in others when the differences are not accounted for.
This becomes very important when knives are thrown in the mix, because of the unforgiving nature of such an attack. Up close there is nothing IMO more dangerous than a short bladed knife: it's hard to see and follow, moves extremely fast, can be concealed until the very last minute by a semi skilled person, and very small motions can quickly cause major damage. Techniques used against an empty hand attack can be used in many instances to defend against a knife wielding person, but the margin for error is infinitesimally smaller. Furthermore, the way your attacker will react to your counter becomes important in some instances where their body will move in a way that becomes an unexpected problem.
Let us use for example a standard mugging scenario taught in many aikido and jujutsu dojo: uke is attacked from behind, tori grabbing the left arm at the wrist or upper arm while the right hand holding the knife either goes around the neck or to the side. We will use ikkyo (arm bar) or an arm lock to restrain uke. Now the basic technique calls for a pull down on the arm holding the knife with the left hand while the head is tucked in towards the arm, sliding under it and ending on uke's side ready to follow through with the restraint technique. Things can be a bit more complicated if uke is holding on to tori's wrist rather than his upper arm, but not overly so.
Now let us examine some the things we might miss if we focus on the large gestures of the technique. If uke were holding the knife to the left side of the neck and used his knee on tori's back knee to collapse him, how does that affect the knife's position in relation to tori's neck, and therefore how would the technique be affected? If the knife was to the right side of the neck with uke's arm laying over tori's shoulder and arm front rather than around the head, such a knee collapse would benefit tori more than uke, as it would drop his neck and head away from the blade and making an opening to slide out of uke's control. I give you another factor: where are uke's feet? Are they controlling tori making it hard to execute the turn into uke's arm needed to get on his side? What about the way uke is holding the blade? Which way tori turns is affected by uke's grip; if using a reverse grip with the arm around the neck things are more "exciting" than if he has a hammer grip on the weapon...
So many small things, yet they can profoundly influence the end result. The lethal capabilities of the knife present dynamics that are not as "final" if things go wrong as with empty hand techniques. Now, I am not advocating you despair if ever confronted by a person with a knife. What I am suggesting is that in your training be aware both physically and mentally of details that can make or break a technique. Work with a partner through variations of grip, stance and foot position, leverage being used against body parts, placement or movement of the knife, etc. Use the basic form of any movement to expand the possibilities of its use, all the while keeping in mind that principles might not change but their application can be tailored to fulfill your needs.
Honestly, if I were faced with a knife and had the chance to get away that would be my first option; of course, there might be the one time where there is no option but to engage and hope for the best. Be aware and meticulously study what has been passed on by your teachers and decide how to best implement it in a realistic way. It can open venues for growth and improvement that you might not have thought of before... because in a knife fight, every edge you can muster counts.
And remember: it is the man that makes the art, not the other way around... so whether you practice karate, aikijutsu, combatives or gung fu it is how you use what you've learned that decides if you go home safe or end up in a world of hurt.