Your eyes should light up when you see a row or column of three in a region, because it is very useful for solving squares. You can immediately use the block to exclude possibilities. In the artificial grid alongside you can see that I've put 1,2,3 in the central region. In the region to the left I've filled in two rows just to illustrate. Because there is this central row the allocations in the right hand region are now fairly fixed even though no squares are allocated. We know that 7;8;9 must occupy the central row to complete row E. However more significantly we can tell that 1;2 and 3 must occupy the top row of this region (in any order though) and 4;5;6 occupy the bottom row. That means by the magic of the block of 3, each square in a blank region is knocked down to only 3 rather than 9 possibilities.
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