Y-Wing

This is an example of a Sudoku grid where we can apply the Y-Wing technique.

If cell A was filled with the value '5'.

And cell B was filled with the value '4'.

This would mean that cell C would have no remaining candidates, i.e. this Sudoku would not be possible.

It's important to note that cells A and B have exactly two candidates, and the candidate we haven't talked about for both cells is '7'.

This means we can remove all '7' candidates from any cell that would cause cells A and B to lose their '7' candidate.

If cell D was '7',

⇒ Cell A would be '5',

⇒ And cell B would be '4',

⇒ Cell C would have no candidates.

This means we can remove the '7' candidate from cell D (and all the other red cells if they contained any '7' candidates).

This is another Y-Wing, but we can remove two candidates here,

If cell A or cell B was a '9',

⇒ Cell C would be a '5',

⇒ And cell D would be a '8',

⇒ Cell E would have no candidates.

Another Y-Wing,

If cell A was a '5',

⇒ Cell B would be a '3',

⇒ And cell C would be a ''4',

⇒ Cell D would have no candidates.

When looking for Y-Wing situations, you are looking for,

Two cells (called 'B' and 'C') with exactly one shared candidate (call this 'X'), and exactly one different candidate (call these 'Y' and 'Z').
A cell ('D') with exactly two candidates 'Y' and 'Z' that can be seen by cells 'B' and 'C'.
We can remove the 'X' candidate from all cells that can see cells 'B' and 'C'.