Whether you are facing a knife or a gun, this attitude is important. You only get one shot (see what I did there?) at it, otherwise the reverse will be true. Your life as you know it, will never be the same.
I think that we practice these things too much, as play. I have 10 tries to get this right. Wait, stop, I missed my grip, let me change this here... It doesn't matter how effective your move is, if you are only playing the move, like we do in training.
To me, the whole point of training knife and gun defense, is learning to go from 0 to 110% instantly, then maintaining that 110% until the other guy is down, and you have the weapon and are safe... all done with control. In the systems I have seen, the techniques used against weapons, are very similar to empty hand techniques. Empty hand is the time to work accuracy, timing, distancing... all those other bits. When you introduce the weapon, its to find out if you can keep all that you learned, at 110%, and when it goes wrong, are you committed enough to finish completely... guy down, you have the weapon and are safe.
If you ever have to use it for real... its not going to go perfectly. You will have to go from 0 to 110% instantly. As soon as you start, one of you will never be the same. Once you gain an extra hole, your time gets extremely limited. You need to adapt, and finish before you bleed out. Gaining additional holes only serves to limit your time. Commitment is key and you can't change your mind once you start. We need to train this way, when we introduce the weapon.
While the use of these techniques against weapons is in reality, pretty low percentage, the attitude and commitment would apply to any self defense situation, where you employ your physical techniques. Once you start, you need to finish, no matter what goes wrong. I believe we respond in the street, the way we practice. Regardless of how effective your technique is... if you only go to 40%... or change your mind, once it starts to go wrong... you are done. You may be done anyway, even if you go at 110% and everything goes perfectly, but hopefully you can take the other guy with you.
Empty hand is about technique. Weapon disarms are about mindset and commitment.