Unique {0} type 4

The cells {3} form a Unique {0} with the values {1} and {2}. There are exactly two ways of placing the values {1} and {2} in the cells of the Unique {0}, forming two possible configurations. In both configurations, each row, column or block touched by the Unique {0} contains each of the two values {1} and {2} exactly once. As a result, if one of these two configurations were part of the solution, it could then be replaced by the other one to get a second valid solution.

Because a valid sudoku cannot have more than one solution, none of the two configurations of the Unique {0} can be valid. Note that the two cells {4} and {5} are the only possible positions of the value {1} in the {6}, thus, one of these two cells will contain the value {1}. Therefore, the only possible way to avoid the two configurations is if neither {4} nor {5} contains the value {2}.