Nine
Pointing pair
A technique Nine explains
A digit can only go in one row (or column) inside a box. Wherever it lands in that box, it is on that line. So it cannot be anywhere else on that line outside the box.
Example
345296178698121278659798246839172498359317576482751
In the bottom right box the 3 can only go in row 8. So the 3 is out everywhere else in row 8. That leaves one cell with only one digit left: 4.
In the app
Nine is the sudoku that writes this reasoning itself, on every puzzle, in the app. The explanation fills in nothing: it names the technique, colours the proof and says why. Free for seven days, then a one-time unlock. No ads, no account, no internet needed.
Everything about Nine
Nine for iPhone and iPad
Nine for Android