This technique relies on spotting two pairs of candidates for a value, and using these to rule out candidates from other boxes.
Take a look at the places where 2 can be for the middle column of blocks
Here they are highlighted:
You can see that they only lie along two lines (columns 4 and 6).
Because the 2s are limited to those positions in the top blocks, it means that columns 4 and 6 are taken. That means that any of the candidates for 2 in the bottom block can be removed from either of those two columns. (It has forced the 2s in the bottom block to be in the middle column.)
You can remove these candidates.
This nicely leaves a single-candidate 7, which you can then fill in!
This technique is reasonably easy to spot because you only need to see candidate pairs in two blocks.