Step 1 · Beginner
What is Hidden Single (only one place for it)? How to Solve Killer Sudoku
When a digit has only one possible cell in a row, column, or box, that cell must be that digit.
What it is
If a digit fits in only one cell of a row, column, or box, that cell is the answer.
Example: look for where 7 can go in a box. Every other cell already has a 7 in its row or column, so only one cell is left. That cell is 7.
How to spot it
Pick one digit and check the boxes one by one. From each copy of that digit already on the board, trace its row and column to cross out cells. The one cell left is your answer.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In box 2, the only cell that can hold a 2 is R2C4.
→ R2C4 is 2.