What's the most complicated sudoku technique that is required to solve the most difficult puzzles?

In other words, for which sudoku techniques do there exist a puzzle that cannot be solved without using that technique? (Brute force checking - just f

In other words, for which sudoku techniques do there exist a puzzle that cannot be solved without using that technique? (Brute force checking - just filling in numbers randomly to see if they work and backtracking if they don't until you find the correct answer - doesn't count as a technique.)

or:In other words, for which sudoku techniques do there exist a puzzle that cannot be solved without using that technique? (Brute force checking - just filling in numbers randomly to see if they work and backtracking if they don't until you find the correct answer - doesn't count as a technique.)

Tags:checking,complicated,