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The Swordfish strategy is an extension of the X-Wing idea. Instead of two rows, we analyze three rows and check whether a given candidate is restricted to the same three columns.
When such a pattern appears, the candidate being analyzed must be placed in these three columns within the selected rows. This allows us to remove it from the remaining rows.
Let's analyze candidate 5 in three selected rows.
All possible positions for digit 5 in these three rows are therefore located exclusively in Columns 6, 7, and 8.
This is our Swordfish pattern. We do not yet know exactly where digit 5 will go, but we know that in Rows 2, 4, and 6 it must be placed in one of Columns 6, 7, or 8.
This is the essence of Swordfish: three rows restrict a given digit to the same three columns, allowing us to make eliminations outside this pattern.
Since the Swordfish pattern uses Columns 6, 7, and 8 for digit 5 in Rows 2, 4, and 6, we can remove candidate 5 from the remaining rows.
These are three valid eliminations that follow directly from the Swordfish relationship.
Let's now see what changed on the board after removing these three candidates.
In advanced strategies, this kind of result is often the most important: we clear impossible candidates from the board and prepare it for the next logical steps.
