How Koga decides
Definition A forcing cycle of checks that harvests material
- A windmill (or see-saw) is a repeating, forcing sequence in which one piece swings back and forth: it gives a discovered check, the enemy king is forced to step aside, the piece swings back to give a direct check, the king is forced back — and on the swing the same piece sweeps up an enemy piece or pawn each time round.
- Because every move in the cycle is a check, the defender has no time to escape: the king is shuffled to and fro while material is picked off for free.
How it is recognised Two matching positions, one piece short
- We follow the main line and take a snapshot of the board after each of the swinging side's moves.
- We look for two snapshots that are identical in every respect except that exactly ONE enemy piece has disappeared between them — the king and our own pieces have returned to the very same squares.
- That return to an identical position, minus one enemy unit, is the tell-tale sign of a full windmill turn: the cycle came back to where it started while pocketing one piece.
Filters Conditions and what does NOT qualify
- Checks make the cycle — between those two matching snapshots the swinging side must have given check at least twice. The mix of a discovered check and a direct check is what makes the king dance and keeps the sequence forcing.
- Repeated direct checks alone are not a windmill: if the king is merely chased without a piece being won on the swing, nothing is harvested and we do not flag it.
- The line must be long enough to contain a full turn — a sequence of fewer than four moves cannot show the position returning, so it is never a windmill.
- We read both sides of the line, so the windmill is caught whichever colour is doing the swinging.
Test positions (the examples that guard against regressions)
Classic windmill on the 7th rank
After Rf7+ the rook unmasks the bishop on f6, which checks the king on h8 along the f6-g7-h8 diagonal. The king must go to g8, the rook swings back to g7 with a fresh check, the king returns to h8, and the rook then swings out again — capturing on c7 and b7 along the way. The position keeps returning while Black loses a pawn each turn of the wheel.
Piece that moves
Attacking piece
King
Targeted / won piece