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 box 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}.