Make sure that your group can be saved, even if either liberty is taken.
... but not from the south. The black stones die, because if Black tries to escape with , White can capture with .
Three examples are inspired by my own games:
After , the white stones are dead. If White plays at a, Black will capture at b. (See shunt.)
Black loses eight pieces, because he lacks liberties.
What is this called? --Anonymous
This state is generally called Shortage of Liberties for which the Japanese word is damezumari which supposedly translates to something like "short of breath". Plays like are popular endgame tesujis that spell ruin for players under 17k. --Patrick Taylor
Patrick Taylor: Thanks to the presence of , capturing at can't save the trapped black stones. If , black should just connect at instead of descending.
Patrick Taylor: After , the marked black stones are caught in oiotoshi and so cannot connect at a to escape atari. If , white can capture at a. If Black plays b, White at c starts the ladder/net along the first line.
Many situations are related to having just two liberties.
In a ladder, an attacker repeatedly reduces a group of stones from two liberties to one liberty.
A simple net keeps the number of liberties to two or less.
White has two liberties. If white moves to either a position, black will reduce white's liberties by moving to the nearest b position.
A double atari can occur when two groups with just two liberties share a liberty that's not an eye.
Every group with just two liberties (not both eyes) can be used as a ko threat.
Auto-atari can only happen when your group had two liberties, you move into one liberty, and you gain no more liberties. In this diagram, I chased after , only to discover...
Same shape, different order.
A snapback is a specific kind of auto-atari.
Alex Weldon: Even three is not enough, quite often.