So here's an example that popped up for me today. I'm belatedly working through Relentless. Here's one of the variations from game 5:

Click Here To Show Diagram Code

[go]$$W Gu Li (white)-Lee Sedol, 2014-05-25, variation
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . . O . X . . . . . X . . . . . |
$$ | . X . O . . . . b a . . . . . X X . . |
$$ | . . . . . . . . . . . . . . . . . O . |
$$ | . . X O O . . . . . . . X . O . O . . |
$$ | . . B X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . X . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . X . . . . . . . . . X O O O . |
$$ | . . . . . . . . . . . . . . . X X O . |
$$ | . . O . . . . . . . . . . . . . . X . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . O . . . . . O . . O . . . . . . |
$$ | . . . . . O . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]

In the game, black played 'a' instead of the marked stone. But if black does play the marked stone, where should white reply? The book recommends a shoulder hit at 'b'. But KataGo goes off in a very different direction:

Yes, I took my time over this. But here is something I noticed today.

Click Here To Show Diagram Code

[go]$$Wc19 Hashimoto-Takagawa 1973-11-08, white to play
$$ +---------------------------------------+
$$ | . . b . . . . . . . . . . . . . . . . |
$$ | d X . X a O . . . . X . . . . . . . . |
$$ | . c X . X . O O O X O . . . . . . . . |
$$ | . O O X X O O X X X . . . . . X . . . |
$$ | . O . O O X O . . . . . . . . . . . . |
$$ | . . O O X X O . . . . . . . . . . . . |
$$ | . O . O X . O . . O O . . . . . . . . |
$$ | . . O X . X O . . X . . . . . . . . . |
$$ | . . O X . X X X . X . X . X . . . . . |
$$ | . . X O O . . . . . . . . . . . O . . |
$$ | . . X X O O O . . O . O . . . . . . . |
$$ | . . . . X X O . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . X . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]

This is a game between Hashimoto (white) and Takagawa in the 13th Old Meijin tournament. At move 72, after chasing the centre group for a while, white comes back and plays the forcing exchanges a for b and then c for d. I'm looking at this and thinking: how do you learn this sort of timing? White could have played those moves much earlier; why is now the right time for it? But KataGo says no, actually, the pro 9 dans did not have a perfect sense of timing. White should have exchanged a for b back on move 37, as soon as this shape appeared on the board.

Full game for anyone who's interested:

I looked at the example and based on my understanding of how the search and neural net interact I suspect you shouldn't read too much into this case, unless you can actually find some variations where failing to make the exchange earlier costs something? Sometimes you can actually find some variations that do show that you have to exchange now. Sometimes there isn't one, where the exchange will always be makeable safely later.

Assuming I didn't miss actual variations that punish missing the exchange (don't trust me entirely, I might have!) then in this case, I would guess that the neural net probably rates the position as very slightly better for white with a stone at "A" and probably the net "understands" that "A" is sente-ish but it's not quite as salient to the neural net if the exchange is not made. So the search on average is returning very slightly higher evaluations for making the exchange earlier, rather than delaying it and having a higher proportion of positions in the tree without the exchange. However, if in all variations the rest of the fight never becomes so forcing as to prevent making it later, then it's at least as good if not better to delay, and the higher evaluations of the neural net for making the exchange is in some sense "fake", it's a small correlated error in the judgment of the relative values of the positions with and without the exchange in positions as being different when objectively they are equal.

Bots will usually refrain from making significantly bad exchanges unless horizon-effecting is happening, but for exchanges that will *probably* only be played in one way (i.e. the optionality of the other choices isn't really of much value), and/or where it's very unlikely that correct play will result in a ko in the intervening time for which the move would be a good threat, for those exchanges there's little to stop them from being preferred. The cost to playing them a bit earlier is very very small or zero, and so some for some fraction of them correlated evaluation noise will have the right sign to outweigh that ~zero cost and give the bot a clear preference to play earlier... and precisely because the cost is very small it doesn't appreciably cost much if any strength in practice for the bot to doing so.

These forcing moves are of the kind which white will be making anyway at some point since they are good endgame. The thing that can change in this position is that black could give up on the cutting stones. From what I learned it is important to get these kind of forcing moves before something changes in the position and they stop becoming forcing.

The forcing moves are also important in the middle game for making eyes. That is the reason I see for why they were played when they were played. Even without this reason it is important to get your forcing moves but for awhile it looked like it was really just endgame. It seems rather farfetched that white could miss out on these as endgames.

In short it feels natural to make the forcing moves before testing if black will give up on the cutting stones. However, it is a bit of a question of what you prefer. Would you like to capture the cutting stones or the corner? I'd rather not have to capture the corner if black can make two equal moves elsewhere (but how likely is that?) and I wouldn't mind capturing the cutting stones. Basically, making or not making the forcing moves both feel natural to me.

We can enjoy having new insight into the minute details of old games with not much effort. For example if you have black play a one space jump on move 55 (those are never wrong) then the issue of when to play the forcing moves appears to come to a head. In this case black didn't defend the cutting stones and we have the typical situation that white would want to make the forcing move before ever capturing the cutting stones. In some other situation capturing the cutting stones immediately is suggested by KataGo as coming before the forcing moves, but with some patience and faith in old Go theory it has so far always updated to the two being about the same.

There is also another reason for delaying forcing moves. It is more important for us that are not the top players It is that you shouldn't be making unnecessary moves. Those moves are a sign of not having known what to do. It is expected that top players, who appear to know what to do, are so meticulous about making the moves that need to be made and not all the other moves. It shows that they work hard to always play better.

It's the increased use of probes that I find most interesting.

There seem to be at least 2 main types:

- the early probe invasion which seems to ask or try to force the opponent's hand in committing to claiming an area, and may also leave possible aji

- the contact probe, which I'd speculate may often be previously researched and potentially opens up a sequence to new variations, or offers an opportunity of a fight or exchange

There's probably a lot of other uses of such moves as well that pass completely above my comprehension.