1Definition: Fins and Sahsimis affect two techniques you already know: X-wings and Swordfish. You can think of fins as imperfect X-Wings or Swordfish. A fin is an extra digit or two where, ideally, they should not be. For instance, an X-Wing with three of the same digit in one of its two lanes, or a Swordfish with a fourth instance of its digit in one of its three lanes.
A Shashimi occurs when a digit is not where it should be in the pattern. In this case the fin may or may not be an extra digit, but it must be in the same block as the cell missing the digit.
A finned X-Wing; it would be a perfect X-Wing of 5's (with conjugate pairs of 5's in columns E and I) except for the "fin" at E3. Because of the fin we can only erase the 5 at D3 (in the fin's block).
2Look For: An X-Wing or Swordfish that almost works; in one lane of the pattern it has a misplaced and/or extra digit(s) in its block.
3Consequence: The regular rules for erasure for the X-Wing or Swordfish can be followed except that the erasure is limited to the block containing the fin.
4Why it Works: It works because, in the context of the fin's block, it doesn't matter exactly where the digit occurs as long as it is in the pattern's lane.
6But we do have a finned X-Wing. We can have one to three 5's in E1, E2, and E3 for it to be a finned X-Wing (if there's no 5 in E3 it's a Sashimi-fin, but that is inconsequential). (But note that a Sashimi-fin with only one 5 on E1 or E2 can also be seen as a Skyscraper, with additional potential erasures.)
7Since the 5's in column I are a conjugate pair, we know that either I3 or I6 will resolve to a 5.
8Finned Swordfish: In this example, which occurs later in the same puzzle, we have a Swordfish for digit 4 in rows 1, 7, and 8,with a fin in block 7. If the fin were not present we could erase all the fours outside the Swordfish cells along the cross-lanes: columns B, D, and G. But with the fin we can only eliminate along column B within block 7: erasing the 4 in B9.
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