A defense mechanism
is an automatic reaction
to a threat
In the case of mechanical warfare, the threat is assaulted when it is within a particular range or certain conditions are met. An overly simplistic example is tying the trigger of a shotgun to a doorknob. Then the door is opened, the shotgun is discharged, eliminating the target.
In the case of human reaction, the defense mechanism is below the threshold of conscious thought. An extended example of this concept is when you touch something cold, the cold receptors in your nervous system send a signal that travels to your brain via your spinal column saying, "left index finger is touching an ice cube." When you touch something hot, the heat receptors in your nervous system sends a signal to your brain via your spinal column saying, "right toe is in warm bathwater."
Should you place your hand on a hot stove, both your hot and cold receptors fire off at the same time. This message goes from your hand to your spine, and the nervous system there relays the message to your brain and automatically forces your arm muscles to rapidly contract in order to get your hand off of the stove.
Whilst I was in a Psychology class, we did an experiment with frozen and hot marbles. You could hold the cold and hot marbles separately, but when the teacher mixed them in a bucket, nobody could plunge their hand into it and keep it there without a very concerted mental effort. Most couldn't keep their hands in it at all.
Should some crazed maniac jump out at you with a knife, your body will generate adrenalin in huge amounts. You will be able to run quickly or fight the attacker, depending on the circumstances. You won't even have to think about block incoming sharp knife. Your hands will be there ahead of your conscious thought, protecting the body.