This technique is based on the fact that all units (rows, columns, and blocks) must contain each digit 1-9. If a candidate can only exist in one cell in a unit, the cell must contain that digit.
Consider the sudoku below.
The cells r4c4 and r5c1
eliminate the candidate 6 from
rows 4 and 5
. Consequently, the only cell remaining in
the right block
that has the candidate 6 is
, so it must contain the digit 6.
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