
Unit Forcing Chains 

A Unit Forcing Chain is a unit where every candidate within the unit is the starting point for a forcing chain that all lead to the elimination of a single candidate. In other words, every candidate in a unit elimates a single value.
Consider the example on this page. All the 6's in column 7 have been highlighted in green. The top 6 is weakly connected to the 6 to the left, which is strongly connected to the 6 straight below, which is weakly connected to the 6 in blue. The second 6 in green from the bottom is weakly connected directly to the 6 in blue. The 6 in green at the bottom is weakly connected to the 5 in the same cell, which is strongly connected to the 5 way over to the left, which is weakly connected to the 2 in the same cell, which is strongly connected to the 2 above, which is weakly connected to the 6 in blue. Since all the 6's in green connect to the 6 in blue, the 6 in blue can be eliminated. This means that no matter where the 6 in column 7 is located, the 6 in blue can be removed.
Note that the first and last connection in each forcing chain must be weak.
Sudoku Snake gives Unit Forcing Chains a skill value of 6500.






