
Finned Jellyfish 

A Finned Jellyfish is a pattern that is one cell short of being a Jellyfish  in other words, four columns that contain a candidate in only four rows, except for one, and vice versa. The logic behind this technique is that if the fin is not true, the rest of the Jellyfish is.
Consider the example on this page. Rows 1, 6, 7 and 9 from the top contain the candidate 8 only in columns 2, 3, 8 and 9, except for the stray 8 in the top left corner (row 1, column 1). If the cell in the topleft corner is NOT an 8, then the jellyfish pattern is true (in this case it makes the topright corner an 8, and the rest reduces to a swordfish. It has the same effect) and you can eliminate all the other 8's in columns 2, 3, 8 and 9. However, if the cell in the topleft corner IS an 8, you can eliminate all other 8's in the topleft box. Either way, the 8's highlighted in green can be eliminated.
Not all fins must be only one cell. Sometimes a fin can extend to several cells that share the same tertiary unit. As long as all pieces of the primary units that are not part of a secondary unit share a unit with a similar candidate that is in a secondary unit, that candidate can be eliminated.
Sudoku Snake gives Finned Jellyfish a skill value of 850.






