Firstly, let’s organise the situation a little bit.
It’s quite obvious that black’s group here only has one eye at the moment.
In order to live, black needs to make another eye somewhere or kill the white group on the left.
Now let’s look at the situation of the white group.
It seems that black can kill the white group by playing at 1.
However, white has a good move at 2.
Black has no choice but to block here to prevent white from making another eye.
White can take the stone and if black plays at 3, white will cut at 4.
It seems like a ko, but because black has no stones at C7, after black takes at A8 white can play at A5 to connect.
Now that we know the white group on the left is alive, let’s look at the situation of white at the bottom.
The bottom white looks very thick, but there are a few points that black can exploit if white goes for the kill.
For example, black has sente at B, C or D and white has no choice but to destroy the eye to prevent black from living.
After that, white has some cut points at A and E that black can make use of.
If black just plays at 1, white will destroy the eye at 2.
Black 3 is also sente and black can take one stone, but white can simply connect at J3.
It’s still just a false eye and there’s nothing much black can do here.
If black 3 plays at F4, white will play at F5. If black G5 white E5 and it will be a similar result.
Now let’s try playing from the other side at at F4.
White 2, 4 and 6 are necessary to kill black.
Black can get the atari at 8 and cut at 9.
Because both A and B are sente for black, white has no choice but to eat at D2.
At one glance, it looks like black didn’t do anything and is still dead.
However…
If you imagine that black has stones at both A and B, then black can play at C to kill the white 4 stones.
This means that if black can play A and B in sente while trying to live or while trying to kill the white group, black has a chance!
Going back to the problem.
We know that through the exchanges from F4, black can cut at D3 eventually.
In order to make it work, black needs to play both A and B in sente.
Now we need to find the sequence that will allow black to play these two moves in sente.
Let’s try playing at B6 directly.
White will live by playing at 2. A and B are miai so white is alive.
If black plays 3 now, white will just protect at 4. The cut at D3 still doesn’t work.
Now let’s try playing at A5 directly.
White will play at 2 and protect at 4.
If black connects at 5 to try to kill white, white plays at 6 and A and B are miai.
White is alive and black is still dead.
So playing at B6 or A5 directly doesn’t work.
We need to find other ways.
Let’s try making an eye at D7.
White needs to destroy the eye at 2 while making an eye for himself.
After white 4 white is alive.
Black can get 5 in sente, but only getting this move in sente is not good enough.
Now let’s try playing at D8.
White can only answer at 2.
If white tries to push at D7, black will also push at 2 and white loses an eye.
Locally black can’t make an eye because B6 and D7 are miai, but…
Black threatening to kill white again at 1 is a good move.
Note that white can’t push at 2 first, because if white does, playing at 4 is not sente anymore.
Black can play at 5 and now even white takes the stone at A, black can play at B to make ko because there’s an extra stone at C3.
White can only play at 1 directly.
Black plays at 2, continuing to threaten to kill white.
White takes the stone.
If black plays at A, white will cut at B.
After black takes the stone white can atari at C and black can’t connect.
It seems that white is alive and black has failed.
However…
Black playing at B6 now is a good move.
Black can live at A next, so white has no choice but to play there.
After white plays at 1, black playing the hane at 2 is another good move.
White’s group is not connected and only has one eye at A, so white has no choice but to play at B.
Now let’s look at the the situation again.
Remember our original objective?
Yes, it’s to play the moves A and B in sente.
And through this magical sequence, black managed to do it!
Now black can come back to the bottom to play this sequence which we showed just now.
All of them are forcing moves considering that white has to kill black.
When black cuts at 9, white’s 4 stones are now in danger as both C3 and D2 doesn’t work.
White has no choice but to resign now.
To summarise, the sequence starting with D8 is the only way for black to live.
This sequence of moves can neither kill white nor allow black to make another eye locally, but through this sequence black can play both A and B in sente, thus allowing the cut at D to work with the help of C.
It’s one of the most beautiful sequences I’ve seen so far.
It’s a pity it was not played out in the actual game.
P.S.
It seems that white (Park Yeonghun) actually saw this sequence as he showed it immediately in the post-game review.
Probably he decided to resign if black played correctly, but in actual game black didn’t see it and resigned after a few moves.
Can’t really blame also because it’s difficult to see it even with ample time, let alone if you are under time pressure.
Easy Go 