Compulsory Capture of Dead Stones
In most situations, the Japanese rules compel players to pass before capturing all strategically dead stones, since playing additional moves to actually remove dead stones would reduce one’s points under territory scoring. For example, consider the following position.
In this corner, black has 4 points (3 from territory plus 1 for the white prisoner). However, if black were to play an additional move here (at the end of the game, when they should pass) to capture the white stone, black would only get 3 points (2 from territory plus 1 for the prisoner).
It is the following rule that allows black to simply remove the single white stone without capturing it, since it is a dead stone within black’s territory.
Article 10. Determining the result
1. After agreement that the game has ended, each player removes any opposing dead stones from his territory as is, and adds them to his prisoners.
However, the fine print of this rule is that dead stones are only removed from territory, and, as we just discussed in the previous post, eyes from groups in seki do not count as territory.
Consider this example from How does this get scored (Japanese)?
The position is a seki, and the single white is dead, but it is not removed and not counted as a prisoner, if black does not play another move to actually capture it. Hence, black should play another move (which should be delayed until the very end of the game, since it is not urgent) to actually capture that white stone, in order to a get net gain of 1 point. Note that there is already zero territory to lose (from self-filling), since it is an eye from a group in seki.
The Japanese rules commentary actually gives a slightly more complicated example to illustrate this concept:
Here, not only should white capture the two black prisoners, black is then compelled to throw in another stone (to prevent the seki from collapsing), which white should then capture again before ending the game. Thus, white should extract 3 points (all from prisoners) from this position.