GNU Go Documentation
9.5.5 Semeai winning moves
A move which is necessary to win a capturing race is called a semeai
move. These are similar to attacking moves, except that they involve
the simultaneous attack of one worm and the defense of another. As for
attack and defense moves, it's important that all moves which win a
semeai are found, so an informed choice can be made between them.
Semeai move reasons should be set by the semeai module. However this
has not been implemented yet. One might also wish to list moves
which increase the lead in a semeai race (removes ko threats) for use
as secondary move reasons. Analogously if we are behind in the race.