A Hidden Single is a cell that contains the only candidate for a digit in a given group/area, but the cell contains other candidates as well.
If only one cell contains a candidate for a certain digit in a given group, no other cells can have that value, therefore the Hidden Single candidate must be the solution for the cell.
If you already used the Single Search and the Snyder Notation strategies, it is unlikely you will find any Hidden Singles.
4 is a candidate only in the yellow cell in the selected row.
4 is a candidate only in the yellow cell in the selected column.
4 is a candidate only in the yellow cell in the selected box.
If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.Sample Puzzle 1 Sample Puzzle 2 Sample Puzzle 2