This has come up in discussion with a few of my students lately, and it once again occurs to me that I've never written it down anywhere. I keep falling back on it, so I'd like to have it here to share. Besides, I think it's useful.
As the title suggests, this is not my model. I got it from Tom Campbell, a kickboxing and PDR coach from somewhere out in Western Massachusetts. It's been well over a decade since I saw Tom, and I have no idea what he's doing now, but he had some excellent knowledge. He's actually featured on a DVD that Blauer Tactical put out called "How NOT to Get Knocked Out". A lot of solid info there.
[Note: This also means that any errors in transcribing this model are entirely mine. I don't wish to put words into Tom's mouth.]
There are four ways to stop any attack.
Obstruct: Put something between yourself and the attack. Unarmed, this means you are sacrificing one part of your body to avoid taking damage to another. The boxer's "shell" or "cover" (where you throw your forearms up in front of your face to stop an incoming punch) is an example of this. Yes, getting punched in the forearm is usually better than getting punched in the face, but you're still taking damage. You're also not providing much of an opportunity to counter attack. On the upside, the timing required to do this isn't particularly complicated. As long as your forearm is in place before the punch lands, you're fine.
Deflect: Redirect the attack so that it doesn't hit you. This is a parry (not PERRY, people). Done correctly, there is less impact than with an obstruction, but the timing is a bit more precise.
Evade: Get out of the way of the attack. There is absolutely no impact here, and, if done well, this can line up a devastating counter. The timing ,however, is much tighter, and done incorrectly, you can end up eating the attack full on.
Intercept: Hit the other person as they are trying to hit you. Not only do you not taking damage, but you deal damage the other person as well. Close to Rory Miller's "Golden Move". Of course, if you flub it, you can trade shots (not great), or just get hit (less great)
The hierarchy runs in two directions:
Order of Efficiency: Intercept-->Evade-->Deflect-->Obstruct.
If you can pull it off, interception is the most efficient thing you can do. Obstructing the attack is the least efficient, but better than getting hit.
Order of Simplicity: Obstruct-->Deflect-->Evade-->Intercept.
Obstructing doesn't require a lot of complex timing. Most people can reliable obstruct an attack within a few reps. Reliably intercepting an attack is tough, unless there is a huge disparity of skill.
This is primarily a striking based model, but I think you could apply it to grappling. Certainly seems applicable to weapons, though I suppose the nature of the weapon would influence the appropriateness of certain tactics.
I find it useful as a way of planning a teaching progression, or of just looking at options when trying to figure out how to deal with a particular attack.