The term "aiuchi" means to hit or strike each other at the same time in kendo (Japanese fencing) and so on. It also has the meaning derived from this that the outcome of a game is not decided.
Aiuchi in kendo
This means that both fencers have an effective hit or strike at the same time during a game. Such a hit or a strike of both the fencers becomes invalid and no points are rewarded. This is called aimen when both the fencers have a men (a blow to the head), and called aigote when they have a kote (a hit on the forearm).
Various stories about aiuchi
Some swordplay schools regard aiuchi as their innermost secret, and such tendency was strong especially in the latter of the Edo period. Most swordplay schools which emphasize aiuchi are branches of the Kage school (a swordsmanship school established in the latter of the Muromachi period). Also, masters of Nippon Kenpo (a Japanese martial art) regard aiuch as an important factor (as one of mental attitudes).
As the Japanese proverb "let your opponent slash your skin to cut off his flesh, and let him cut your bone to break his bone marrow" (you should be prepared to sacrifice your own body so that you can defeat your opponent) describes, the idea that the fencer himself or herself should win and survive is always behind the concept of aiuchi in swordsmanship schools. Some cases which are actually not aiuchi appear to be aiuchi (which means aiuchi has something similar to a cross-counter in boxing). It is said this is because there is always a slight difference in ability between fencers or fighters even if they are called a master. The story about Mitsutoshi YAGYU is one actual example of that, and the movie "Shichinin no Samurai" (Seven Samurai) also quotes this story.
There is a legend that an archery master and a gun master killed each other due to aiuchi in the Sengoku period (the period of Warring States).
One story about a duel in the Edo period says that since both fighters bled too much to survive due to a long fight, they bravely killed each other by stabbing the opponent's throat at the same time by mutual agreement (this way of thinking was the same as kenka-ryoseibai [where there is a quarrel, both parties are to be punished], and people had intentions and considerations that both would not leave any feelings of regret or resentment). This is an example of the intentional aiuchi to death.
Needless to say, it is hard to achieve aiuchi when an opponent consists of many people.
Aiuchi is one of the favorable scenes in Western mythology and folklore (such as Beowulf), and creative works such as monster movies and cartoons.
Persons died from aiuchi
The grandfather of Tadaaki ONO
The younger brother of Hirokado TSUKUSHI (strictly speaking, there was a slight time lag)