Y-Wing: the pincer of three cells
The Y-Wing, also called XY-Wing, is one of the extreme techniques. Three cells with two candidates each act like a hinge and force an elimination in a fourth cell, with no guessing. Click through the pattern below.
Interactive example
Click ‘Step’ to see the pattern and the elimination.
Try it: Y-Wing
How the Y-Wing works
You need a pivot cell with two candidates, say 1 and 2. Plus two wings that see the pivot: one holds 1 and 3, the other 2 and 3. Whatever value the pivot takes, one of the two wings is forced to be the 3. So any cell that sees BOTH wings already has a 3 within reach and cannot be a 3 itself. The 3 is removed there. The pattern is called a Y-Wing because the three cells meet like the arms of a Y.