When exactly two candidate numbers are in exactly two cells on a line or block, any other numbers can be removed from those cells.

In the example, the red cells has hidden pair. Hidden pair may be difficult to spot (as the name suggests), but it's easy to apply once you have found the two cells. Can you solve this very hard Sudoku?

Finned wing

Finned wing is a restricted case of a general wing technique, where a candidate number can be removed like in wing but only from cells seeing both the fin cell and the corner cell of the wing.

In this example, do you find which cell can be filled in the fin block?

Sashimi wing

Sashimi is yet an another wing, where a fin corner (see above) is already filled.

In the example, the red cells indicate a wing with a sashimi on the top left corner. Which cell can be filled once sashimi prunes its another candidate number?

XYZ wing

XYZ wing consists of three cells with three common numbers. The number which is common to all XYZ cells can obviously be removed from cells seeing all the XYZ cells.

In the example, which number is common to all XYZ cells, and which cell can be filled after XYZ wing is applied?