When there are only 2 candidates left for a constraint, one of them must be true and the other must be false. In bifurcation, both branches of the fork are tried by placing the candidate in its cell and solving the puzzle from that point forward. When a contradiction is found, the remaining candidate can be placed.
Bifurcation has a bad reputation in the Sudoku community because it relies on backtracking and looks similar to guessing. Many advanced solving techniques were introduced to reduce the need for bifurcation.
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