Two String Kite

Crane is the third in a trio of Sudoku solving techniques that consist of two strong links, and one weak link.

If you're not familiar with Strong and Weak links, you may find it useful to read about those first.

In this Sudoku, we have a sequence of Strong-Weak-Strong links.

There is a Strong link between cells A and B on the '4' candidate.

There is a Weak link between cells B and C on the '4' candidate.

There is a Strong link between cells C and D on the '4' candidate.

If we assume the '4' in cell A is false,

⇒ The 4 in cell B is true.

⇒ The 4 in cell C is false.

⇒ The 4 in cell D is true.

Separately, if we assume the '4' in cell D is false,

⇒ The 4 in cell C is true.

⇒ The 4 in cell B is false.

⇒ The 4 in cell A is true.

This means that the value in cell A or cell D (or both) must be '4'.

If the value of Cell A is '4', we would remove all '4' candidates from these cells.

If the value of Cell D is '4', we would remove all '4' candidates from these cells.

If the value of cell A or D (or both) must be '4', this means we can the '4' candidates where these two areas overlap.

There will always be two cells where these areas overlap, but only one of them could contain a candidate we can remove - if the cell in the column 9 had the value '4' as a candidate, the link between cells C and D would be a Weak link instead, and there wouldn't be a Two String Kite here at all.