How Koga decides
- The move captures an enemy piece (not the king).
- That captured square is guarded — at least one enemy piece could recapture on it.
- But every one of those guards is pinned, so none of them can legally recapture. The piece falls off for nothing.
- This is more specific than a plain pin: the point is not just that a piece is frozen, but that its paralysis lets us win the piece it was supposed to protect.
- An absolute pin (the guard is pinned to its own king) always counts — recapturing would be illegal.
- A relative pin (the guard shields a more valuable piece) counts only when recapturing would still lose more than it gains: the piece behind the guard must be worth at least as much as the piece we captured with. If recapturing and then taking our capturer would come out ahead for the defender, the guard is not truly tied down and we do not fire.
- The pinning piece must already be on the board before our move. If our own move is what lands the pinner into place, the guard was free to recapture beforehand, so this is not a pinned-defender win.
- We judge the pins on the position right after the capture — which also means a file or diagonal that opens only once our piece vacates it can supply the pin.
- When the same move is both a pinned-defender capture and a discovered attack, Koga keeps whichever describes the larger material gain.
- If the captured piece is worth more than (or as much as) the piece the discovery hits, the pinned-defender label stays and the discovered attack is dropped; if the discovery wins more, it is the other way round.
- Victim — the guarded piece we capture and keep.
- Pinned defender — the guard that should have recaptured but is frozen.
- Pinner — our long-range piece that holds the defender in place.
- Behind — the king (absolute pin) or more valuable piece (relative pin) the defender is pinned to.
Test positions (the examples that guard against regressions)
Capturing a knight guarded only by a pinned pawn
From a real game. The knight on c6 is guarded only by the d5 pawn. Right now the d-file is blocked by Black's own queen on d6, but after Qxc6 that queen is gone and the d7 rook pins the d5 pawn to the d4 queen. The pawn cannot recapture, so the knight is simply won.
Pinned-defender absorbs the plain pin and capturing-defender labels
Same position as above. Because the pinned-defender story already explains the whole point — the d5 pawn is pinned AND it was the c6 knight's defender — Koga reports only Pinned Defender and suppresses the bare pin and the capturing-defender label that describe the same idea less completely.
Knight guarded only by an absolutely pinned knight
A constructed position. The bishop captures the d5 knight, whose sole guard is the c7 knight. But the c7 knight is pinned by the c8 rook to its own king on c1, so it cannot recapture — the d5 knight is won outright.
Rook takes a knight that a pinned knight cannot recapture
Lichess 00bQA. Rxe7+ grabs the e7 knight. Its only guard is the g6 knight, but that knight is absolutely pinned by the g5 rook to the king on g7, so Nxe7 is illegal. The e7 knight is won.
Guard exists but is not pinned
A constructed position. The pawn captures the d5 knight, which the c7 knight guards. Nothing pins the c7 knight, so it would simply recapture. With a free guard available this is not a pinned-defender win.
One of two guards is free
A constructed position. The pawn takes the d5 knight, which has two guards: the c7 knight and the e6 pawn. The c7 knight is pinned by the c8 rook, but the e6 pawn is not — it can recapture freely. Because not every guard is frozen, Koga does not fire.
Taking the pinned piece, not the piece it guards
A constructed position. The bishop captures the e5 knight, which is itself pinned by the e1 rook to the e7 queen. We took the pinned piece directly rather than something it was guarding, so this is an ordinary attack on a pinned piece — not a pinned-defender win.
The move itself creates the pin
Lichess 0TDKH. Bxh6 captures on h6, and only afterwards does the bishop now on h6 line up against the g7 bishop and f8 rook. Since the pin did not exist before the move, the guard was free to recapture, so this is not a pinned-defender win.
Relative pin that does not really hold
A constructed position. The queen takes the d5 knight, guarded by the e6 pawn. The pawn is in a relative pin (the e1 rook eyes the e8 rook behind it), but recapturing only exposes a rook, worth less than the queen we captured with — the defender would happily make that trade. The pin is not operative, so Koga does not fire.