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Double Exocet in sudoku
Two valid Exocets whose base pairs sit in different boxes of the same row or column, each pair holding the same four candidates between its two cells.
Why it works
The four base cells share one row or column and hold only those four candidates, so between them they take all four digits. Those digits are therefore eliminated from every other cell of that row or column, in addition to each Exocet's own eliminations.
How to spot it
After finding one Exocet, check the same row or column in another box for a second base pair with the same four candidates.
Implemented, but never needed: an easier technique always reaches the same result first.
In a puzzle's rating
Each time the solver needs Double Exocet, the puzzle's difficulty rating grows by 480. Its class is Extreme. How the rating works.
More last resort
Exocet · Pattern Overlay · Forcing Chain · Digit Forcing Chain · Nishio Forcing Chain · Cell Forcing Chain · Unit Forcing Chain · Forcing Net · Brute Force