On this page https://senseis.xmp.net/?BigSacrificeToGetAWall only the paragraphs about the 19 point trick play and the 4-4 ogeima enclosure seem correct. Are there other similar examples?
When I review my games with Katago I sometimes see sequences with big sacrifices, I find them fascinating. How does one learn to play like that? I can't find a spectacular example at the moment but here is a position from one of my games, I was Black and taking handicap. Black made some mistakes on the top left and the top right but still has a good game.

Click Here To Show Diagram Code

[go]$$Bc Black to play
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . X X . X . . . X W . X . O O . . |
$$ | . . X X X X O . . X O . . . . X . . . |
$$ | . . . O X X O . . . . . . . . . . O . |
$$ | . . . . O O . . . . . . . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . O . |
$$ | . . . . . . . . . . . . . . . . O X . |
$$ | . . . , . . . . . , . . . . . O X X . |
$$ | . . . . . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . X . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O O X . |
$$ | . . X . . . . . . . X . . . . X O X . |
$$ | . . . . . . . . . X O X . . O O X . . |
$$ | . . . X . O . . O O O . . . O X . . . |
$$ | . . . . X O . . . . . . . . O . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

In the game I played a, White extended and then I played b. But Katago suggests to play b directly.

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . b . . . . . . . |
$$ | . . . X X . X . . . X O . X . O O . . |
$$ | . . X X X X O . . X O a . . . X . . . |
$$ | . . . O X X O . . . . . . . . . . O . |
$$ | . . . . O O . . . . . . . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . O . |[/go]

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . 3 1 . . . . . . . |
$$ | . . . X X . X . . 2 X O . X . O O . . |
$$ | . . X X X X O . 5 X O . . . . X . . . |
$$ | . . . O X X O . 6 4 . . . . . . . O . |
$$ | . . . . O O . . . . . . . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . O . |[/go]

Probably White wants to give Black a few points in exchange for a strong central position, in order to attack the weak group K8. But what if White plays at ?

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . 2 1 4 9 . . . . . |
$$ | . . . X X . X . . 3 X O 6 X . O O . . |
$$ | . . X X X X O . . X O 5 7 . . X . . . |
$$ | . . . O X X O . . . 8 . . . . . . O . |
$$ | . . . . O O . . . . . . . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . O . |[/go]

Click Here To Show Diagram Code

[go]$$Wcm10
$$ ---------------------------------------
$$ | . . . . . . . . . 8 . 1 3 2 . . . . . |
$$ | . . . . . X . . . 4 O X O X 9 . . . . |
$$ | . . . X X . X . . X X O O X 7 O O . . |
$$ | . . X X X X O . . X O X X 5 . X . . . |
$$ | . . . O X X O . . . O . . 0 . . . O . |
$$ | . . . . O O . . . . . 6 . . . . . . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . O . |[/go]

(This looks like a book position but I can't remember where I saw it.)

I don't know how to learn this. I remember Guo Juan has really good lectures around the shape that occurs in your example. It starts out differently but the final technique is the same. She calls these middle game joseki, never really heard anyone else use that except as an offhand. You can learn these kind of things as memorization but you need to be in a special zone to see this kind of stuff without contemplating about it if it is not something you memorized. If you want to get into this kind of zone then you probably need to start with something more elementary, for example ladders and that might make go more fun because there is nothing more fun then catching someone in a ladder and do to that you need see them in advance and verify them without giving it away by zig-zagging your head or triple checking

In your example, the hane is actually a counter-hane and this is really the normal response to hane. Crosscutting, connecting and ataris are all moves but the stronger you are the more counter-hane you will encounter.

Your position:

a is definitely the normal local move, and b is normally bad for territory. in this case B is too weak in the centre on the whole right side of the board, so b becomes viable and even good to settle quickly.

When kvasir says counter hane is strong, I think he is right when fighting for space in the centre. In this case the value is more about how to rescue weak stones without giving up too much. I.e. don't get cut off into a bigger weak group.

Sacrifice variation:
Play 19 before 17 is of course better normally though perhaps it doesn't matter much here. It does lose a ko threat though. is at K14.

This shape appears in joseki for both

3-4, low approach, shusaku kosumi 4-5, 3 space low extension, invasion, attach underneath
and
4-4, low approach, knight's move defense, 3 space low extension, invasion, attach underneath

The 4-4 variation is locally better for the 4-4 side, and I think the 3-4 variation is slightly better for the 3-4 side. It also seems slightly worse for B to have the stone high.

Tips on sacrifice

• Only sacrifice when you are weaker locally. If you are stronger, you should make it difficult for the opponent to get anything. When weaker, generally consider sacrifice at one point to get stronger everywhere else if possible by focusing on the opponent's weakest point.
• Sacrifice to gain strength from forcing moves. This is important if you have other weak groups to save nearby.
• Sacrifice a group (generally on the side) which you have enough to support to counter-surround even if the opponent surrounds you. If the opponent can't even surround you and you simply save your weak group, that isn't even a sacrifice, as in your 3rd diagram.
• sacrifice tesuji only occur if the opponent plays weak stones to surround your weak group.

Sacrifice strategy
If you want to strategically lay traps for the opponent to get them to try to take your stones on too small of a scale, then defend your weak groups but not as solidly. Use this sort of extra large oogeima rather than one and two space extensions. Add flexibility to your group when choosing direction of play in fights elsewhere on the board to get strength in the direction of where your weak groups might have to escape to if threatened, I.e. counterintuitively at the least valuable escape point since that is where the attacker will push you towards. Or supporting cutting the opponent's attacks.

Of course, this won't help much if your opponent kills you despite your support or if you add another overconcentrated move to live. Speedy strategies require some skill to manage as they dont secure valuable connection/eyespace/territory as well, so the player must balance this aji with developmentinto the centre. The opponent may be content to use the aji to break into the centre potential with a strong group. This is what b is trying to avoid, compared to a which sacrifices your 4th line stone.

The reason you normally sacrifice the side for the centre is that although the centre isn't very valuable, it is still very open. If you can get many connected moves in a row, you don't need the small eyespace that the side provides and the opponent may also find it difficult to reduce. However to get this, you must have some kind of group on the side that you are sacrificing. If it is already dead, you can't get many forcing moves.

I can't really comment on this position, but one sacrifice strategy that my teacher has brought up several times is to sacrifice stones after they have already served their purpose.

For example, you cut in the center and both players extend, extend, but then the opponent goes back and connects their 2 group underneath. Now the cutting stones are no longer making an attack and the opponent has already spent a move to fix their shape. You can get extra forcing moves by making the opponent surround your cutting stones.

I recognize that the previous replies are from strong players who know this basic tip. But I am writing because I hope this is a helpful reminder for kyu players like me.

Yes, what you say corresponds to part of Baduk Doctor's proverb "the value of stones is always changing". (Conversely kikashi stones that are useless for now may serve a purpose in the future). But that's not the question I was asking, I was wondering how to create a group with the intention of sacrificing it immediately. On the other hand, as kvasir points out, the technique may be too fancy for my level, better learn to see ladders in avance instead of being repeatedly annoyed by cutting stones that can't be captured in a ladders for instance.

I may post interesting positions in this thread when I encounter them in the future though.

How to learn this?

Play 10 games with only 1 focus: at any point in the game you considere a big sacrifice to get a strong central position. At some point you will execute it. And you will then review with KataGo if the decision and the timing were right, or at least justified. You don't care about the result too much, just focus on the timing and execution of the sacrifice.

After 10 games or so I would expect to have gained some understanding about the theme.

Also, it's not something you can do yourself. It's an offer from you to your opponent and they can refuse. It might also be something your opponent offers.

So I can see it taking many games to even have this exchange work.

What I kind of meant is that the move needs to be on the player's radar or they can never play this sacrifice sequence. Maybe one way to play more such sacrifices is to consider different moves.



I'd add that it is possible to go further than this. You can completely revamp your playing style and play 100s of games in the new way. I have changed my style a few times, usually in minor ways, and I think it is a good experience. You can do it because of different reasons. One reason is to learn something new but other reasons are that you might genuinely be a tougher player in some areas and that you might simply find it more enjoyable to play if you change your approach.