How to solve a finned Swordfish in Sudoku
A finned Swordfish is a Swordfish with an extra candidate (a fin) in one box, which still clears the digit from that box.
How it works
It is a Swordfish with one or more spare cells in a single box. Either a fin holds the digit, which clears the box, or none does and the Swordfish works as normal.
Either way, the digit can be removed from the cells in the Swordfish's columns that lie in the fin's box.
How to spot it
It shows up as an almost-Swordfish, with one extra cell that shares a box with part of the pattern.
A worked example
This is a step from a real puzzle, a moment where the next move needs a finned Swordfish.
F are the fish cells, R are the fins (the extra cells, all in one box). Struck-through digits are the ones that can be removed.
5 is almost limited to 3 columns across 3 rows (R3C4, R3C9, R4C3, R4C4, R7C3, R7C9), with the extra cell R7C7 (the fin) in box 9. If the fin is 5, it clears 5 from the rest of box 9. If not, the 3 rows use up 5 in those columns. Either way, remove 5 from R9C9.
Where it fits in Rocksled Sudoku
Puzzles that need a finned Swordfish are never rated below Expert. Each step of this kind adds 88 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.