I'm no martial arts expert and I've never been in a knife fight, but I'm in good shape, fast and not afraid at all of the sight of blood or invoking self-defense violence when needed. I think I could defend myself pretty well with a super sharp folding combat knife. Even though it only has a four inch blade, half of that is serrated and the point is wicked sharp. Enough penetration to reach the heart, lungs, etc if the guy doesn't have some serious man boobs going on. Even the fattest of people have very little over their breast bone, which can easily be penetrated by a knife with a very sharp point and some force behind it. Even though my blade is "only" four inches, we're not talking about needing 18+ inches of penetration like we do with handguns because we're not going to be stabbing through an arm bone to reach the vitals. Stab, twist and RIP sideways when pulling it out. Wash, rinse, repeat as needed.
Seriously, it's not like human anatomy is that hard to figure out. If I'm ever attacked and need the knife, I'm pretty sure that I'll be facing them "full frontal" when I need to bring it into use. Sure, the initial attack may come from behind, but when/if I engage, I'm probably going to be facing them directly. If they turn away and/or run, so much the better. Aim for the central/upper right area of the chest and you're going to do some major damage. Same for the soft neck areas. Even a stab/slash at the trachea is going to put the major hurting on an attacker even though it may not be immediately lethal. I'd also aim for the eye sockets. Even if I don't directly hit the eye, the knife has a good chance of sliding into the eye off the concave ocular bone socket, especially if I hit them on either side of the upper nose area just under the eyebrow. If I don't actually hit the eye? It's still going to cause a LOT of bleeding which will greatly hinder their vision. However, if I'm grabbed from behind, that arm(s) are going to get a LOT of damage. I'd also move my pelvis to one side or the other and begin stabbing. Just maybe I'd hit the BINGO spot for a male attacker. I'd also try stabbing up and behind over whatever shoulder I had access to and maybe hit him in the head. Either way, if I make contact, it's going to hurt really, really fukin' badly for that dude.
The key is to STAB and reach something vital. I agree slashing isn't as effective as it's mainly a pain and "oh my God lots of blood" deterrent unless you get lucky and cut some wrist tendons or the like. I'd also avoid any stabbing much below the breast bone as gut stabs aren't going to be incapacitating any more than gut shots with a pistol are. Sure, they may die later, but it's going to take time.
