XY-Chain
Expert Removes candidates Used in: Sudoku, Killer Sudoku
A chain of bi-value cells where each shares one candidate with the next. The same digit sits at both ends, one of which must be true, so it is eliminated from any cell that sees both ends of the chain.
How to spot it
A chain of bi-value cells, each sharing one candidate with the next, that starts and ends on the same digit.
What you can conclude
Eliminate that digit from any cell seeing both ends.
Worked example
A chain of {2,7}–{7,9}–{9,2} cells ends on 2 at both ends; a cell seeing both ends cannot be 2.