How Koga decides
Idea Block first, then strike
- We already have a threatening move available — call it the strike. Played on its own it does not yet win, because the opponent has a single saving resource: a square to flee to, or a defender that guards the key square.
- Blocking means first planting one of our own pieces on exactly the square that resource depends on. With that square occupied, the saving resource is gone, and the strike now wins.
- So the move we play is quiet in itself — its whole point is to take away the one square the defence relied on.
Mate by suffocation Sealing the escape square
- One pattern: our piece sits on the only flight square the enemy king would have used. We then deliver a check that, without that flight square, is checkmate.
- We confirm this literally: after our blocking move, if we could move twice in a row, the strike would be mate. The flight square is sealed by our own piece, so the king has nowhere to run.
Cutting a line Interposing on a defender's line
- The other pattern: an enemy long-range piece — a rook, bishop, or queen — is guarding the square our strike wants to use. We drop a piece directly onto the line between that defender and the key square, even sacrificing it.
- Once the opponent's guard is severed (typically by capturing our interposing piece), the strike lands on the now-undefended square and wins material.
- We only count this when the strike actually achieves something — it captures, gives check, or wins material on the square. A quiet relocation onto that line does not qualify.
When we look The strike must already be there
- The strike has to be a move that is already legal in the starting position — blocking removes the defence to a threat that already exists, it does not invent a new one.
- We read the blocking move and the strike out of the engine's main line, so the payoff that justifies the block can sit a couple of moves deeper in the sequence.
Test positions (the examples that guard against regressions)
Bishop seals the king's escape, then mate
A constructed position. Our bishop goes to d6; on its own this is quiet, but it covers f8 — the only flight square the black king on g8 has. With f8 sealed, the queen on h5 can take h7 and it is mate, because the king can no longer run to f8.
Piece that moves
Attacking piece
King
Targeted / won piece
Rook sacrifice cuts the bishop's defence
A constructed position (a Fischer–Benko shape). Black drops the rook on f3 — square that lies on the white bishop's g2–e4 diagonal. After the bishop takes on f3, it no longer guards e4, so the pawn push to e4 hits both the bishop on f3 and the knight on d3 and wins material.
Sacrificed piece
Defender (removed or overloaded)
Piece that moves
Targeted / won piece