A nozoki (term borrowed from Go) is a forcing move to which opponent has just a single answer to defend against an immediate threat. This situation occurs if one threatens to cut a connection between major groups or key stones of opponent's. The stone causing this threat is called a nozoki.
Sometimes it's hard to tell if a move is forcing, i.e. if the nozoki is correct or wrong. Playing correct nozokis is one of the most sophisticated aspects of hex strategy.
In this position red player can play 2 correct nozokis - c4 or c3. In case of playing c3, blue has to defend with c4. In case of playing c4, blue has to defend with c3. Otherwise blue loses the game.
Notice that moves on a4, a5, f2 and f3 are wrong nozokis. Against a4, a5 blue has a choice of playing b5 which improves their position. Similarly, against f2, f3 blue should play d2.
There is a great use for playing nozokis. In certain conditions a player can benefit from them and claim victory. Look at the following example:
The only winning move for red is the nozoki on d6. If red misses it the game proceeds as follows and the winner is blue.
If d6 is played then red win.