This technique consists of checking if there are two cells in a UNIT that have only two equal numbers as possible solutions for both cells (bi-value cells, composed of the same two numbers or, using the definition, the same candidates), if this occurs, these two numbers can be eliminated from the other cells, because they can only be in these cells, although their actual position is not known.
In this example, in box 1, the numbers 1 and 3 will be in one of the positions marked in yellow, if we choose the number “1” in cell A2, cell B3 will necessarily contain the number “3” and vice versa:
Therefore, the numbers “1” and “3” can be removed from the other cells in box 1:
Another example, in a column:
