X-Chain
Advanced Logic28 / 32One digit, alternating links: the ends cover everything between.
A chain built from a single digit, alternating strong links (only two spots in a unit) with weak ones (sharing a unit), starting and ending on a strong link. Walk it from either end.
Assume the first end is false: the strong link forces its partner true, the weak link passes falseness on, and so on, until the far end comes out true. So one of the two ends is always true, and everything seeing both ends is eliminated.
See it in action
Step 1 of 4
The setup
Map a digit's strong links. Try to walk from one strong link to another via cells that share units, alternating strong and weak: the longer the walk, the more surprising the elimination. Step through this real position to see it happen.
Practice
Follow the X-Chain and remove its digit from the cells seeing both chain ends.
Drill 1 of 29Tap a cell, then the candidate digit you want to remove.