Thanks, Cataholic, you beat me to the snopes link.

If you're in knife fighting range, (as in closer than you'd like to be), and cannot escape, use your knees. You have far more strength in your legs than you do in your arms, and your knees are close to an attacker's groin, which is almost an instantaneous disabling blow. Also, if you swing with your arms, the attacker can easily grab your arms and immobilize you. There is no such thing as a fair fight.