Among other drills, we were going through knife defense and learned one to defend against a downward stabbing attack. My initial feeling was that it put my left arm at great risk to a cut, so I mentally categorised this defense as 'dangerous' and best avoided. I raised this point with Conan whose comments (as usual) made a lot of sense. If attacked by someone with a knife (assuming that running away isn't possible), you will get cut anyway if you do nothing. A defensive move, counterattack and disarm must be made with total commitment. You may still get cut, but probably much less so, and most importantly, it gives you a good chance to stop the attacker from harming you further.
So my thinking about defense against any weapon, particularly a knife goes along these lines...
I hope to be always aware enough of my surroundings to avoid a 'situation'. If despite that, I'm fronted by an armed assailant, my best option would be to get away. If I can't get away, I will try to diffuse the situation if at all possible, but I will immediately look for any suitable object within my reach that could become a defensive weapon in my hand. I will arm myself with any useful object as soon as possible (excepting in the unlikely case that I can talk the attacker out of his plans and have him hand over his weapon).
Having come to this point, I'm committed to taking action, fast and with determination, as the only alternative is to be a guaranteed victim. If I have been able to arm myself, I first of all use my weapon to disable the attacker's weapons hand (The Kali concept of defanging the snake) while staying out of the reach of his weapon. If I can't locate a defensive weapon, I will be even more concerned with staying out of his reach, but I may need to block/parry a strike, while going for a simultaneous counterattack. So whether I have disabled his weapons hand with my weapon or whether I've blocked and grabbed his hand, I need follow up immediately, or better simultaneously. The goal of my follow up is to disarm the attacker and to ensure that he is unable to continue his attack, be it with weapon, with strikes or kicks.
No defense is without risk, and in the sudden attack any practiced response will be less than perfect, but as long as it's done with that total commitment, it will lead to a better outcome than doing nothing or making a half-hearted move. A cut to the outside of my forearm and a disarmed and disabled attacker is infinitely preferable to a deep stab wound to the body followed by further cuts/stabs delivered by the attacker.
I am sure that in the sudden confusion of a real attack, things won't be so clear, and choices limited. After all, an attacker doesn't want a fight, no, he wants to hurt me to gain advantage, so he will only attack if circumstances are in his favour. With the added advantage of surprise, the odds will be in his favour.
All my training does is to even the odds a bit. Learning to use a stick and a knife effectively means I'm better prepared to use a weapon to defend myself. Learning moves to defend against a knife attack and knowing which parts of me are most vulnerable is as important. Sure, in the heat and trauma of a sudden attack, I won't have time to go down a long decision tree to work out the correct response. But drills are about positioning, minimising exposure and setting up a counter attack as much as they are about specific grips. So in the muddle of things when things go bad, the part of the brain that deals with threats will likely remember a move from the drills which will vastly increase my chances of survival. Moves can create distance or close distance. Other drills and sparring practice have increased my reaction speed, my ability to function under pressure and to read the attacker's body language. Finally, training ensures that I know how to disable an opponent, be it on the ground or standing up.
Nothing guarantees my survival in a sudden armed attack. Training improves my chances. Realistic training improves them further. But in the end, my ability to react appropriately and efficiently and my will to survive will be what most strongly determines the outcome.