XY-Chain
super-tough
This other kind of forcing chain creates pairs in which a single number is eliminated amongst shared cells. Consider the below partial puzzle.
If r1c1=1, then clearly all the cells for which I’ve crossed out the 1s cannot have 1s. But the same situation occurs if r1c1=3, due to the chain of cancelations.
If r1c1=3, then r1c5=7, r2c6=6, r2c8=8, and r3c9=1.
Again, the same cells are shared by r3c9, so we can cross out the same 1s in either case.