This week, actually, a week before, a white belt wanted to review a knife disarming technique during the review part of class. The owners have been debating adding in weapons disarming techniques into the fundamental classes. My practice partner was overly enthusiastic to practice the new technique. Personally, I thought practicing knife disarming technique at white belt level a bad idea but I went along with it. I know in order to disarm someone with a weapon, it takes a lot of training. My first reaction to a knife attacker would be to flee. I think that trying to disarm someone without a good reason is dangerous.
Anyways, I wanted to test him. I put the pretend knife in my hand and approached with a downward motion. He weakly grabbed the downward hacking wrist and then did a terrible kimura grip.
On the second attempt, I was going to pretend more like a knife attacker. With the knife, I came at him. He weakly grabbed my wrist, I immediately escaped the grip, and I pretend to stabbed him in the back five times. Right afterwards, I asked him what he excepted would really happen? He didn’t answer. He appeared to realize the reality of the situation, in truth, a mistake could be fatal.
journeyjjbluebelt 
If the attacker knows how to yield that knife, you are in big trouble no matter who you are or how much jiu jitsu you know. I once read somewhere, you have a better chance of surviving swiming in a tank full of sharks than survivng a knife attack where the attacker knows how to yield the knife. Best knife defense, go get a gun.