In this level of reduction,In a block if two cells have the same
set of possible values (i.e) if the values are repeated among the
cells. Then those possible values can't be present in the
possibilities of any other cells of that block.
Note :
The above concept is also applicable in the case of n cells with n
number of possibilities
In this block,two cells have the same set of possible values {2,5}. Therefore these values should be removed from the possible values for other cells in that block.
In this case, the block is having the values {2,7,9} {7,9} and {2,9} in three cells repeatedly.Therefore these values can't be there in possible values of other cells in the block.
Figure: A Rectangular-Block from 9x9 sudoku.
Consecutive Sudoku Algorithms
Greater Sudoku Algorithms