Six logical techniques used by solvers. Listed from easiest to hardest — each technique page details the condition, principle, example, and caveats.
Basic Elimination
Exclude the 8 cells around a cat and the same row, column, and region. If a region has only one empty cell left, place a cat there.
Row/Column Lock
If every candidate in a color sits in the same row (or column), exclude that row (column) in other regions.
Naked Pair
If two regions' candidates both occupy exactly the same two rows (or columns), exclude those rows/columns in other regions.
Naked Triple
Same as Naked Pair but with three regions on three rows/columns.
X-Wing
When a color occupies exactly two rows forming a 2-column rectangle, eliminate that color's candidates in those two columns on other rows.
Swordfish
A 3×3 extension of X-Wing — cross-elimination over 3 rows and 3 columns.