
This is almost a swordfish. If you look
at the three yellow columns,
If the blue cell didn't contain a 1, then candidate 1 would be
confined to just 3 rows in those three columns.
But this isn't the case. The pesky blue cell ruins our
swordfish. So the blue cell is the fin. Our
target cell is now r9c4 (with the slashed 1) because it resides in
the box with the fin (box 8). So consider just two
possibilites:
 The blue cell is a 1. This would
eliminate the slashed 1 in r9c4
 The blue cell is NOT a 1. Now we would
have a nice valid swordfish which would also remove the slashed 1
So either way, the slashed 1 can be removed.
(Why are there yellow cells that don't contain a 1? click here)
More swordfish information 
