Sue-De-Coq was introduced by one of the forum member who was known under the nickname Sue-De-Coq. Originally this method was known as Two-Sector-Disjoint-Subsets, but soon users started to use the name Sue-De-Coq.
To use this method follow the following steps:

  1. Look for two or three cells within the same block and which are also aligned in the same row or column. The number of candidates in these cells should be equal to the number of cell plus two or more. So for two cell you should have at least 4 candidates.
  2. We also need to find at least one cell which is located on the same row or column of the previous mentioned two cells. This cell should have at least two candidates in common with the above mentioned two cells.
  3. We also need to find a cell located in the same block as the two cells mentioned under (1) and should have at least two candidates in common. However these two candidates should not be equal to the candidates found under (2).
  4. Remove all candidates which are equal to the ones found under (2) and which are located in the same row or column. (1)
  5. Remove all candidate which are equal to the ones found under (3) and which are located in the same block. (1)

The following two examples will clarify the above rules.

First Example:

Example1 sue-de-coq

In this example we have found the cells D8 and E8, which are both located in same block and are aligned on the same row. The two cells contain the following candidates: 2-3-5-8 (total 4 candidates). The two cells comply with the first rule.

When we search in row 8 we will find the cell B8 with the candidates 2-8 and searching the block will give us cell F7 with the candidates 5-3.

It is clear that the two cells D8 and E8 will have two candidates from the values 2-3-5-8 as the solution. Which two is unknown. These are all the possible combinations from the values 2-3-5-8:
2-3, 2-5, 2-8, 3-5, 3-8, 5-8

The combinations 2-8 and 3-5 are not possible, because this will eliminate all candidates from cell B8 or F7. All the other combinations are possible. This means that there will always be a 2 or a 8 in one of the two red cells and a 8 or a 2 in the green cell. This will eliminate all values 2 and 8 from row 8.

Also candidate 3 or 5 will be the solution in the red cell and also in the blue cell. This eliminates all candidates inside this block.

Second Example:

Example2 :sue-de-coq

The two red cell contain the candidates : 3-6-7-8-9 (total 5 candidates). To use the method Sue-De-Coq the number of involved cells should be equal to the number of candidates. This means we have to find three more cells.
One of those cell can be found in the same column and contains the candidates 8-6. These values may not appear in the other cells!. Inside the block the two blue cells are found containing the candidates 3-7-9.

We know that the solution of the red cells can be found in the values 3-6-7-8-9. Which combination is yet unknown. The following combinations can be obtained from these values:

3-6, 3-7, 3-8, 3-9, 6-7, 6-8, 6-9, 7-8, 7-9, 8-9

The combinations 8-6 and 7-9 can be eliminated and also the combination 9-3 and 7-3. If the solution for red cells is the combination 7-3 then this would mean that cell D1 9 would be and cell F1 would also be 9.This is not possible, so the solution 7-3 (and also 9-3) can be eliminated. This will leave the following combinations:

3-6, 3-8, 6-7, 6-9, 7-8, 8-9.

It doesn't matter which combination you will use, but in all cases there wil be a 8 and a 6 present in the red and green cells. This means that all candidate 6 and 8 can be eliminated from the column.

The same applies to the red and blue cells. The solution will always be a combination containing the values 7-9-3. This eliminates all the candidates 7-9-3 from the cell in the same block.