White creates a ko
The solution is to hane at ; black has to play
in order to make two eye shapes; the throw-in at
then starts the ko. This is a two-stage ko that is favourable for Black, in which where Black can immediately capture, forcing White to find the first threat.
If white tries to play on the top first, black will just hane and make two eyes (white can't push at a or black will immediately capture).