X-Chain
Expert Removes candidates Used in: Sudoku, Killer Sudoku
A chain of strong and weak links on a single digit. Because the two ends cannot both be off, the digit is true at one end — so any cell that sees both ends cannot hold it.
How to spot it
A chain of alternating strong and weak links on a single digit.
What you can conclude
Eliminate the digit from any cell that sees both ends of the chain.
Worked example
Follow 9 along strong and weak links; the two ends cannot both be empty, so any cell seeing both loses its 9.