Block and Column / Row Interactions
Sometimes, when you examine a block, you’ll notice that a particular number is forced to a specific row or column. This doesn’t reveal where the number goes, but it can help you cancel out the number from other squares
Take a look at the diagram below. We can see that in the upper-left block, the number 7 must appear in the middle row. This means that the 7 cannot appear in any of the square I’ve marked with an asterisk. In a more complete puzzle, this might be enough information to identify where the 7 must go in on or more of those lower two blocks.