Voiceless, what do you think of the following scenarios:
1) Standing in front of a knife guy, pushing yourself off of him (chest), take a big step (3 ft) and face opposite direction, running moderately and controlled. If he tries to run after you and you have created enough distance, you will simply elephant-stomp him in the stomach (small elephants are known to give sort of a backwards kick and it can be deadly). In other words, a straight backwards kick. Why this approach? Well, our sensei has developed this new method. He says "this way, the attacker can't get close enough to me. He can't grab me because my leg will be kicking him and he will cripple to the floor from the kick. He would have to get close enough to be able to grab me, but he can't, because my leg is protecting me with the backwards kick." This approach worked for one of our club members who got ran after by a drunk guy. He kicked him in the stomach with the backward kick and they had to carry the guy away back to the bar. Of course this works for a drunk, fighting-ignorant guy, but for a vicious trained maniac.. I doubt it. This is why our sensei and us are always open to opinions from the outside. Do you think this would be a good approach? My opinion is that there might be a problem with the guy cutting one of your leg arteries.. I don't know.
I'm not quite sure what kind of kick you mean, maybe something like this one:
In general my advice would be if you've reached "If he tries to run after you and you have created enough distance,"
: Keep on running. If you have distance, he can't harm you, so why close the distance again¿ I recommend rather to work on widening the distance, bring obstacles (cars, fences, trash cans, houses etc.) between you and the attacker.
Now that of course depends also from the situation. Other than in a competitive sportive fight in a self defensive situation the objectives between those involved differ. While in a sportive fight both opponents want to overcome the other guy an win the fight, in a self defensive situation the victims primary objective is to stay unharmed while the attacker(s) want(s) to harm the victim and
stay unharmed himself. Surviving a life threatening situation is usually a bigger motivator than the prospect of harming someone (unless it's needed to survive yourself). So with somewhat equal athletic skills, the victim will mentally be able to run faster and longer than the attacker. If however you have the feeling that the attacker is faster and has better cardio and would not give up before you gas out, then you should do something before as long as you're still able.
That back kick is somewhat problematic, because if you miss he is in your back and you don't have your arms ready to defend yourself. And as you mentioned there is the possibility of him cutting your leg arteries. So, facing him rather try to kick him in the groin as you would try to drive your shin through his pelvic bone.
But again, that all depends on the situation.
2) You are standing in front of the knife attacker and just all of a sudden, you run backwards, but with fast great steps, still facing him. After those steps you run the hell away. Supposedly the attacker would be that dumb that he would be surprised for that second that you take steps back. He would be like "huh?" in his mind and that would supposedly be enough for you to get the hell away.
I would rather disadvise running backwards. You'd be slower than him running forwards (if he doesn't go "huh?") and you wouldn't see any obstacles you could stumble over.