Sudoku Technique
Forcing chains are advanced sudoku logic for boards where ordinary patterns are not enough. They are not blind guessing; they are a structured way to prove what must happen.
A forcing chain begins with a candidate or bivalue cell and follows the consequences. If one branch leads to a contradiction, the starting assumption is false. If two branches lead to the same candidate placement or elimination, that result is forced.
Very sparse boards can reach positions where the next step is not a clean rectangle or wing. Forcing chains give expert solvers a way to keep moving while still using logic, especially on 17-clue sudoku and other extreme evil puzzles.