Swordfish: the 3x3 pattern
The Swordfish is the big brother of the X-Wing: instead of two rows and two columns it works across three. A digit that appears in three rows only in the same three columns can be removed everywhere else in those columns. Click through an example below.
Interactive example
Click ‘Step’ to see the pattern and the elimination.
Try it: Swordfish
How the Swordfish works
Find a digit that appears as a candidate in three rows only within the same three columns. Two of the three columns per row is enough, it need not be all three. Together the cells span a 3x3 frame. However the digit distributes across those three rows, it ends up filling exactly one of the marked rows in each of the three columns. So it cannot sit anywhere else in those three columns and can be removed there. As with the X-Wing, the same holds with the roles of rows and columns swapped.
More techniques
X-Wing · Skyscraper · XY-Chain