Swordfish:

A swordfish is a 3x3 version of an X wing
(2x2).
Swordfish: three
rows where a candidate is confined to three columns. (or vice
versa)
This swordfish is defined by the 6's in the rows 1, 6, and 7. So
the target
cells are in the columns.
The confusing aspect of the Swordfish is that the candidate in question
does not have to be present in all nine cells of the Swordfish.
In rows 1, 6, and 7, candidate 6 is confined to columns 3,4, and 5. This meets the definition of a swordfish.
The yellow squares are shown here to illustrate the
3 by 3 nature of the swordfish. But notice that the 6 is missing
from r1c3, r6c5, and r7c4. This still meets the definition of a
swordfish (6's are confined to three columns in those three rows).
Candidate 6 is now committed in rows 1, 6, and 7 as
well as columns 3, 4, and 5.
Any other 6's in those columns can be eliminated.
More swordfish information 
