is the key move (although it fails). Examining where it succeeds and then the one refutation will lead to the solution.
If black captures with , white starts a ko in the corner to capture three stones. Black cannot connect because of oiotoshi.
This , preventing the throw-in, results in ko again. Although defending with
at
is no longer oiotoshi, white will then play
at a and it is double damezumari yet again.
If black plays to prevent the ko,
results in double damezumari, and thus captures the
stones.
Preventing the ko with doesn't work either. White throws in with
, and after
it is double damezumari again.