Sudoku Tutorial
Finned Jellyfish
This is almost a Jellyfish. If you look at the four yellow rows,  If the blue cell didn't contain a 1, then candidate 1 would be confined to just four columns in those four rows.

But this isn't the case.  The pesky blue cell ruins our Jellyfish.  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:
  1. The blue cell is a 1.  This would eliminate the slashed 1 in r9c4
  2. The blue cell is NOT a 1.  Now we would have a nice valid jellyfish which would also remove the slashed 1
So either way, the slashed 1 can be removed.

You might notice that some of the yellow cells don't contain a 1.  Why is this?

The 16 cells are highlighted in yellow to show the 4x4 nature of the jellyfish.  It is not necessary that candidate 1 be present in all 16 cells.  What's important is that the candidiate 1 is confined to just four columns in the four selected rows.  (Because of the fin, the 1's are present in 5 columns in those 4 rows)

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