Block / Block Interactions
When a number appears as candidates in two different blocks, in two squares each, and those squares occupy the same two rows or columns, they monopolize those rows/columns and can be eliminated from the third block that they share
For example, in the partial Sudoku puzzle below, the cells with question marks are the only cells in their respective blocks that can contain a 3. This accounts for all the 3s in columns four and five. As there can be no other 3s in those two columns, we can rule-out 3 as a candidate in the cells with an asterisk.
The rows and columns involved don’t necessarily need to be adjacent! In the example below, the question marks reveal the only squares in their respective blocks that can contain a 2. This accounts for all the 2s in rows four and six, so 2 can be eliminated from the remaining cells in those rows, shown with asterisks here.