Hidden single
A value that fits in only one cell of a unit goes there
How to use it
Flip the question: not "what can this cell take?" but "where can this symbol go in this line?". For each row and column, and for each symbol the line still misses, mark the empty cells that could host it. If a symbol has exactly one possible host, it goes there — even though that host may show several other candidates.
Hidden singles are the workhorse between naked ones: they fire when a symbol is squeezed by givens spread across two crossing lines. Fertile ground: nearly-complete lines, and the symbols they still miss.
Why it is sound
Each symbol must occur exactly once in every row and column. If all cells of a unit except one exclude the symbol (by row/column conflicts or a used pair), the remaining cell carries it in every solution, whatever else that cell's candidate list says.
A worked example
Where can the digit 2 go in row 2? r2c1 is closed (its only letter is C, and the pair C2 is already used at r3c2), column 2 is closed (2 sits at r3c2), column 3 likewise (2 at r4c3): every empty cell of the row excludes 2 except r2c4. The digit has a single home, so it lives there, even though r2c4's own candidate list showed more options before the count. The mirror question — "where can this symbol go in this line?" — finds deductions that naked scans of the cells never surface.