Unique {0} type 2
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. The only possible way to
avoid these two configurations is if one of {4}
contains the value {6}.
Other occurrences of the value {6} can therefore be removed
from the cells that share a row, column or box with {5}.