Oiotoshi is a Japanese term from the game of Go (Wei Qi / Badouk). Literally meaning "chase and let fall," when applied to Go, it is usually translated as connect-and-die.

In Go, a group of stones is captured if it has no liberties left; that is, if none of the stones in that group has an empty point adjacent to it. A group is said to be in atari if it has only a single liberty. This is somewhat analogous to "check" in chess (although, in general, losing a group does not lose the game). Unless it is possible to capture some of the surrounding stones in one move (that is, unless some of them are in atari as well), the only way to escape atari is to play on the one remaining liberty. This can work if that point has at least two liberties (if it has only one, the group will still be in atari and the opponent can capture immediately) or if playing there will result in the endangered group being connected to another group of friendly stones.

"Oiotoshi" arises when that second possibility is fulfilled (that is, a player whose group is in atari can play on the remaining liberty in order to connect to another group), but the other group has only two liberties, including the shared one. Since playing on the shared liberty removes it, and the original group has no other liberties (because it was in atari), the resulting group will have only a single liberty, and is thus in atari, and all the stones (both groups) can be immediately captured, hence "connect-and-die." Here is an example (White is O, Black is X, edge of the board is ###):

............
............
..XX......O.
.XOX.XXXX.O.
.XOXXOOOXXO.
.XOO.OO.OOO.
####a##b####

White has three groups here. The one on the left is in atari (its one liberty being shown by the empty point marked with an "a" underneath). The middle one has two liberties (a and b), one of which it shares with the left group. The one on the right may be considered alive for our purposes. If White connects the left and middle groups by playing "a," Black will just play "b" and capture all 10 stones. White could play "b," saving the middle group and probably inducing Black to capture the other four stones by playing "a," but there is little benefit to doing this immediately since if Black captures at "a," White can always play at "b" at that time. Rather, White should probably hold "b" in reserve as a ko threat.

Depending on the circumstances, it is sometimes possible to weasel one's way out of oiotoshi. The only way to do it (aside from using it as a ko threat and having the opponent ignore it to win the ko) is if you can arrange is so that the act of increasing the liberties of the second group threatens something bigger than the value of capturing the group in atari. Since you're threatening a big follow-up, the opponent can't capture your stones in atari, and must answer somewhere else, at which point you can connect the endangered stones. However, it's often impossible to arrange such a thing (in the example above, for instance), so in most cases, one must simply bite the bullet and abandon any stones caught in oiotoshi.

Like snapback and damezumari, setting up an oiotoshi is often a tesuji for solving a tricky tsumego (life-and-death problem).

Thanks to sekicho for the Japanese translation.

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