Fight to live through a ko
If a group cannot make two eyes unconditionally but CAN make one eye plus a ko, it is 'alive by ko' (劫活): the opponent must win the ko to kill it, which costs moves and ko threats elsewhere. Ko life is far better than dying for free — it forces the opponent to spend resources, and if your ko threats are bigger, the group may even live. A special case is the 'corner bent-four' (盘角曲四): by an old theorem, with no ko threats it is dead after the game ends — so never leave one sitting if you can avoid it.