Sudoku Strategy

1. The X-Wing

The X-Wing method is one of the most basic advanced Sudoku strategies. It can also be applied in some intermediate levels, although its incidence is very low in these cases. Its goal is to eliminate candidates.
The player can use this strategy when there is one candidate repeated in four cells that form a square or rectangle when mentally connected by row and column. By making an X linking diagonally the two opposite extremities of this rectangle, the player finds only two possible sets of positions for that digit.
The next step will be to test those sets on the grid and eliminate the digit from any cell that would become impossible in both situations.

2. The Swordfish

This strategy helps to eliminate a candidate from cells too. To apply it, the player must find a digit candidate to two cells in the same row, in three different rows. These cells must also be united by column, regardless of the shape they create.
Connecting them will result in a closed chain that reveals two sets of possible placements for that digit. The player can then put them to the test and eliminate that number as a candidate to the cells where it would become impossible in both cases.

3. Forcing Chains

Forcing chains is one of the easiest advanced Sudoku strategies to understand. Unfortunately, applying it requires very high levels of concentration and the outcome can be uncertain. When it does work, it gives the player the solution for one cell.
To apply a simple forcing chain, there must be cells with only two candidates in the grid. The player picks one and begins testing the changes to puzzle when applying each of the two digits in that cell. The goal is to find if there is a cell that would bear the same result whichever digit is used. If so, that will safely be the solution for it.

4. The XY-Wing

The XY-Wing is a strategy to remove candidates. It can be applied when there are three cells in the grid, each with only a pair of candidates that share at least one digit among them (e.g. AB/AC/CB). With a bit of mental effort, the player can picture a Y when connecting them, with one cell working as the stem and the remaining as the branches.
The next step is to trace lines in each row and column of the cells to form a square or rectangle. If any of the shared digits are candidates within the lines connecting the cells or at an intersection point, they can be safely removed.

Source: sudokuonline.io

Video