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.