Fight to live through a ko
When two eyes are impossible, the fallback is one sure eye plus a ko — alive by ko. The attacker cannot kill without winning that ko first, and winning costs ko threats spent elsewhere on the board; if your threats are bigger the group lives outright, and even losing the ko trades for plays somewhere — never a free death. Treat corner bent four (盘角曲四) as a liability from the start: left alone, it turns dead the moment the ko threats run out. Here the top-left eye is already secure, and the second eye hangs on the marked white stone, down to its last liberty — capture it and White must spend a move elsewhere before recapturing, which is the whole mechanism of the ko (1.6, 6.10). Black to play: start the ko for life.