Capture two, recapture one
The phrase capture two, recapture one describes the sequence below. Note that this is not a ko.
There are a lot of situations where this sequence is a possible variation, the consequences of which are easily overlooked. An example:
Black to live. The combination of the marked shape with the 1-2 point is very common in life and death problems. Yet our desire to atari may overcome us:
This is a line easily visualized and often played by both players if they fail to see the meaning of this shape.
Black's correct move is the 1-2 point. White can't play at a due to shortage of liberties. If White next b, Black a gives life.
See also: