Sudoku Technique
Swordfish is an advanced sudoku technique for removing candidates across three rows or three columns. It is rarer than X-Wing, but it matters on hard and evil sudoku boards that stay crowded deep into the solve.
Search for Swordfish only after simpler techniques have stalled and your candidate notes are reliable.
Pick one candidate number. If that candidate appears in two or three cells across three different rows, and all of those cells sit inside the same three columns, those rows and columns form a Swordfish pattern. The candidate must be placed somewhere inside that pattern, so it can be removed from other cells in the three columns.
The pattern also works in reverse: three columns can lock a candidate into the same three rows.
The most common mistake is forcing a pattern when the candidate appears in a fourth column or row. If the candidate is not restricted to exactly the same three crossing lines, the elimination is not safe.