W-Wing

This is an example of a W-Wing.

If the value of cell 'A' was '4', this would mean that cell 'B' would not be '4'.

If the value of cell 'C' was '4', this would mean that cell 'D' would not be '4'.

If cell 'A' and cell 'C' had the value '4', this would leave row 2 with no remaining '4' candidates, i.e. an illegal state.

If cell 'E' had the value '7', cell 'A' and cell 'C' would have the value '4', leaving row 2 with no remaining '4' candidates. This means we can remove the '7' candidate from cell 'E'.

This is another W-Wing. If cell 'A' has the value '4',

⇒ Cell 'B' will be '9' ⇒ Cell 'C' will not be '9'.

⇒ Cell 'D' will be '9' ⇒ Cell 'E' will not be '9'.

This leaves column 7 without a '9' candidate, i.e. an invalid state.

In this example, there is only one candidate that triggers this chain of reasoning.

This chain of reasoning applies to all of the red cells, i.e. We would be able to remove any '4' candidates in the red cells.

Another W-Wing. If cell 'A' has the value '9',

⇒ Cell 'B' will be '4' ⇒ Cells 'C' and 'D' will not be '4'.

⇒ Cell 'E' will be '4' ⇒ Cell 'F' will not be '4'.

This leaves column 7 without a '4' candidate, i.e. an invalid state.

In this example, there is only one candidate that triggers this chain of reasoning.

This chain of reasoning applies to the other red cell, but that cell already has a value on this occasion.

When looking for a W-Wing, you are looking for two cells that,

Are in a different row, column and region.
Have the same two candidates.
If both cells were set to the same value, would cause that value to be removed entirely from a row, column or region.

If you can find two cells that meet these requirements, you want to look for candidate(s) in a different cell that would trigger that chain of events; you can remove those candidate(s).