How it works? Straight to an example.
Find a cell with 3 candidates (F6). In row (F) of this cell find a cell outside the box (box 6) that contains two or three candidates that are in cell F6. In column (6) outside the box (Box 6) of this cell (F6) find a cell that contains two or three candidates from this cell (F6). The two cells of the “clamp” must contain all candidates in use from the center cell. Candidates marked in red can be deleted as candidates marked in green form a locked set.
There are few authors who show this technique, difficult to have sudokus with its application.
Curious about the theory see at: https://www.sudokuwiki.org/Fireworks.
