Create the ko shape
When a group cannot be killed cleanly but the attacker cannot be refuted either, the fight resolves into a ko: one capture that can be answered by one recapture, forbidden for a single turn only. Making the ko is the weaker side's weapon — instead of dying outright, you turn the whole board into collateral: every ko threat you own becomes spendable, and if the opponent's answers run out you win everything back. Count threats before starting, because the side with more and bigger threats effectively owns the ko. This shape — a white stone in atari, ringed on three sides — is the classic ko seed. Black to play: take, and open the exchange.