Region Forcing Chains
Consider each possible position of a digit in a row, column, or block and derive conclusions shared by all resulting chains.
All candidates have been entered in the empty cells.
At least one candidate in A is true. B marks the conclusion shared by every branch. On a solid link, one false end makes the other end true. On a dashed link, one true end makes the other end false. Apply these rules in link order in each branch. All 3 branches make B true. B is true for every possible A.