The Group Bound algorithm helps you identify candidates which can be elliminated due to a restriction placed by one group onto another. For example the fact that a candidate can only be in two cells for a given row, which also are inside a region, means that the region can only have that candidate in those two cells.
When groups overlap, they can impose candidate restrictions on each other, if the candidate only appears in the overlaping cells.
4 is a candidate only on row 1 in box 3.
That means that candidates 4 can be removed from all other cells on row 1 that are outside of box 3: cell [1,6].
7 is a candidate on row 7 only in box 7.
That means that candidates 7 can be removed from all other cells in box 7 which are not in row 7, cells [8,3] and [9,3].
1 is a candidate in column 6 only in box 8.
That means that candidates 1 can be removed from all other cells in box 8 which are not in column 6: cells [9,4] and [9,5].
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 3