And in many real situations with such an attacker, you'll never have a chance to. You won't know it's an attack until the guy walking past you is suddenly stabbing you.
Knowing how to defend yourself is always better than not knowing, but the sad truth is, knife attacks are very, very hard to defend against for many, many reasons, a big one being surprise. Knives are very concealable; I remember a buddy of mine hiding a kitchen knife with his bare arm and giving everyone a nice surprise by suddenly screaming and stabbing it into the table we were sitting at...
![Small Eek](images/smilies/smallstick/smalleek.gif)
Smaller knives are still quite deadly. Your standard prison-yard-style shanking is pretty hard to defend against, if you don't know to expect it right then.