In Japanese hand-to-hand combat and martial arts, atemi designate blows and hits, by opposition to twistings, strangleholds, holding technique and throws. Karate is a typical martial art focusing on atemi. Some strikes against vital parts of the body can kill or incapacitate the opponent: on the solar plexus, at the temple, under the nose, in the eyes, genitals, or under the chin. Most traditional Japanese martial arts (the ancestors of judo, jujutsu and aikido) do not focus on atemi very much, since they were supposed to be used on the battlefield against armoured opponents. Atemi is the study of nerve and pressure points. This knowledge helps by knowing where to hit your opponent. It can also be used for types of therapy similar to accupuncture.
Atemis are, however, often designed with little more than distraction in mind. Whilst an atemi can certainly damage an opponent, the original concept was of a trailing blow, to abdomen, eyes or other painful but not fatal area of the body, so as to open the way up for a more damaging technique - a throw, wrist lock etc which might be resisted were the opponent to have his/her wits about him/her. Originally from ju jutsu (jiu jitsu, ju jitsu), atemis have been rejected (judo), played down (most main stream aikido) and made the sole focus (karate) of those martial arts originating from samurai combat systems. Unless specifically designed to kill or maim, an atemi isn't really designed to knock out or damage the opponent in most cases of use. Nor is it a primary technique, being very much secondary, in both intention and execution, when used according to its original intention of opening up, or covering up, more dangerous options.