Backfire: How Not to Set Up Your Opponent's Winning Move
In 26,000 AI-vs-AI games, there is a move that was played 2,047 times. The win rate of the side that played it: 0%. Not a single win. It is a move that sets up the opponent's winning move with your own hand: the "backfire".
Bombs on the board
In bocch you can't choose your own numbers. They change by themselves according to how many stones are next to you. Color doesn't matter, so your opponent's numbers change with your moves too.
When your opponent already has one stone across, any opponent stone that "would reach if its number were one different" is a bomb. One too small is gap +1; one too large is gap −1. As a practical rule of thumb, be suspicious first of opponent stones with a 3 or 4 that have come close to your home.
A bomb doesn't go off by itself. It goes off when someone moves next to it. And the one who lights the fuse is usually you.
Landing backfire: danger next to your landing square
In the position below, it is White's move. Black already has one stone across, on 4a.
Black's 5e has the number 3 and is 4 rows from the far row. It is one short: a gap +1 bomb. White has 18 legal moves, and 2 of them light the fuse.
If White plays 3d→5d, White lands next to 5e and 5e's number becomes 4. Then 5e→1a gets Black's second stone across, and Black wins. 3d→5f does the same thing, because 5f is also next to 5e.
Leaving backfire: danger next to your starting square
The next position is also White's move, and Black has one stone across, on 6a.
Black's 5d has the number 4 and is 3 rows from the far row. This time it is one too many: gap −1. Three of the four stones making up 5d's number are white, so White is the one holding up Black's bomb. White has 10 legal moves, and only one is dangerous.
If White plays 5c→2c, 5c becomes empty and 5d's number drops to 3. 5d→2a wins for Black. White jumped far away, but the fuse is lit the instant it leaves.
The moment you block is the most dangerous
What makes backfires scary is that they look like blocking moves. In the diagram below, Black's 3c has the number 2 and is 2 rows from the far row: a reach. White tries to stop it with a "push-up", placing a stone next to it to raise its number.
3c is stopped. But on the other side of the landing square 4b sat 5c, with the number 1 and 2 rows from the far row (gap +1). White stopped one and created another.
The same thing happens with a "pull-away", where you move a neighboring stone away.
The more you focus on blocking, the less you look at the stones next door. The moment you think "got it, it's stopped" is the most dangerous moment of all.
Only two places to check
Once your opponent has one stone across, check just two places before you move. You don't need to read the whole board.
- Is there an opponent gap +1 stone on the 8 squares around where you are going?
- Is there an opponent gap −1 stone on the 8 squares around where you came from?
Practice problem
White to move. Black has one stone across, on 2a. White has 18 moves, and 2 of them light the fuse. Which ones?
Answer
The bomb is Black's 6b. It has the number 2 and is 1 row from the far row: one too many, gap −1. What holds 6b up is White's 6a and 6c.
The answers are 6c→5d and 6c→6d. Both take 6c away from 6b, so 6b drops to the number 1, and 6b→5a gets Black's second stone across.
The same 6c is safe if it moves to 5b or 5c, because it stays next to 6b. 6a also holds 6b up, but wherever 6a moves, it lands next to 6b, so the number doesn't change. Even for a stone holding up a bomb, whether it goes off depends on where that stone moves.
Summary
- Once your opponent has one stone across, look for bombs (opponent stones with gap +1 or gap −1).
- Gap +1 next to your landing square is a landing backfire; gap −1 next to your starting square is a leaving backfire.
- The more a move is meant to block, the more you should look next door.
This article is a written version of episode 22 of the YouTube series "bocch定跡研究". The positions are taken from actual AI-vs-AI games.