1. When you are not the instigator, you didn't provoke the other person verbally or physically.Originally Posted by Spiegel
2. When you or someone else is at risk of great bodily harm, such as getting shot, raped, stabbed, killed, etc....
3. When you or the person threatened has no reasonable means of escape.
If all three of these conditions are met, go for the eyes and push towards the brain! Or grab the throat hard and pull at it as if you were ripping out a plug, it's easier than punching the throat because there's some pretty tough armor there even if it doesn't seem like it.
P.S. Or use common sense when you know you can't win, like if they have a gun aimed at you point blank.