X-Cycles are similar to Singles Chains in their use of links between similarly numbered candidates within a unit (row, column or box).
Links are classed as Strong Links
(dark green) if the candidates they connect are the only two of that value in the unit, or Weak
(light green) if there are additional candidates of the same value within the unit. A group of such links becomes an X-Cycle if they
join together to form a complete loop. An X-Cycle is subject to Rule 3 if it meets the following criteria:-
It contains an odd number of links.
There is exactly one example of consecutive weak links.
All strong links occur in groups having an odd number of members.
This puzzle contains such a loop for candidates of value 2. If you examine the loop you will soon convince yourself of the truth of Rule 3
which states that the candidate at the junction between two weak links must constitute a non-solution and may be removed.