When someone is using a knife, he gains range, speed, ease of distraction and destruction…
There is no magic. There is a formula. For an attacker a successful preemptive attack equals to needing to bypass the defender's reaction time and efficiency. This is the element of surprise.
For a Defender surviving means train your reaction time and efficiency and train to apply them methodically. This is the element of not being surprised.
You just need to train methodically more in Pure Krav Maga until you feel comfortable. You need to keep the principles in devising your training drills.
Everything has its limits including reaction time and awareness and the time and effort you are willing to spend.
You must know your limits. The rest is luck. The meaning of Timing is knowing the limits of reaction time and acting within the limits, on time!
The more your training time is structured, the better it helps you to keep track on what you covered and what you need cover and the more efficient becomes the time you spend.
This is the simplicity of Pure Krav Maga which is in fact it is very Complex!
When you are using two hands and two legs and a good skilled experienced judgment and find a way to be more efficient than your opponent you win.
But to become efficient you need to consider the worse advantage your attacker might have and find a solution.
You need to also speed up the training to the point they do not work, then slow down and find why, and increase the speed again.
Common sense. In reality it is not like in the Martial Art movie where Jean Claude Van Damme is fighting an Arnis guy. The idea is to realize what is going to happen before and stop it from happening.
In addition the idea is to evaluate which side of the opponent will be in reach when he gets closer and how you will reach his pressure point and not get the knife all in a split second.
As for sentry removal, surprise attack, he wants to learn how to surprise you , and you want to learn how to be alert and not get into the point you are too late to react effectively. Bad timing of the day? Bad luck?
One way to deal with is to generalize and say why train in knife defense if knife is fast and very hard to block. I should either get a knife or die without wasting my free time training.
Another way is to try to find what is the worse scenario, break it down and devise methods that their logic works on all levels of a matrix. Will it work all the time? did you train right or enough? Lets hope so!
In most countries you are not going to carry a knife on you. If you were to hang out in the country where a knife is allowed without getting arrested, you would probably have it, and if you could have a pistol, you might have it as well.
But if someone is showing you a knife because he wants your money or your family, then you have a quick decision to make. To give up or to fight and win, even if it means losing your life, but you could lose your life if he kills you after anyway, so the fact that he did not kill you immediately, gives you a warning that you better do something and get in control of the situation instead of letting someone control your body, property and perhaps life.
Let's say you were standing as a guard and someone tried to kill you by surprise from behind. Maybe a woman was approaching you from the front and got too close waiting for the moment you take your eyes of her if you are distracted to pull a knife and stab you, you probably got the knife the second you turned your head away...
So as you get closer to a potential danger you need to keep your awareness more alert, and you need to keep enough distance from a potential dangerous person, so if he tries anything you will have time to see it and react on time.