A stun is a sharp, powerful blow that causes a short-term, temporary interruption in the attacker's physical attack and thought process. It does not cause long-term injury to the attacker. Stunning is used as a quick, simple way to slow down an attacker. A stun only last a few minutes, cause bruises and sore muscles at worst, and may be apologized for easily.
Stuns do not even have to make contact to work. A quick, focused punch just short of the nose, with an accompanying loud kiai, will cause the opponent to snap his or her head back and will momentarily stun the opponent.
Counter-attacking is what most people consider self-defense to be, but, as discussed above, it may not be necessary to counter-attack, you may be able to evade the attack and run. Many times, it is best to evade an attack, stun the attacker, and then run.
The first three defensive actions (evading, blocking, and stunning) do not prevent the attacker from continuing his or her attack. Counter-attacking permits control of the attacker and the situation. Control may be gained by use of a restraining hold, choke, strangle, or a strike. A strike to the head that causes disorientation is a form of control. The purpose of a counter-attack is to stop the attacker's initial attack and to prevent further attack.