From Sudopedia, the free Sudoku reference guide

From Sudopedia

Here is a forcing net technique that I have used to solve a number of fiendish puzzles. First consider two cells A and B which are conjugate pairs in X, but not necessisarily bivalue cells. Obviously if A is X then B is not X and vice versa. This is a one link forcing chain. Next starting with A=X I try to find an inplication path that forces B to be X. If one exists then B is X, otherwise I start with B=X and try to find an implication path that forces A to be X. If one exists then A is X. Otherwise the technique does not work on this pair of cells, but it may still work on another pair of cells in the puzzle. I also use this technique on a cell C which is one of 3 or more candidates in a house. In this case, if cell C=X forces one of the other cells in the house to be X, then X can be eliminated from C. Perhaps a good name for this technique is a forcing wrap. Essentialy this is a single digit wrap technique even though the implication path may be mostly XY chain links. Nienhaus, July 19. 08.