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Nishio Forcing Chains
From sudokuwiki.org, the puzzle solver's site
|A Nishio forcing chain is sometimes the exact reverse of a Digit Forcing Chain. If you look at the first diagram on that page you'll notice the first 'type' in the set of four ways a contradiction can be found is a destination candidiate that is ON when the start candidate is both ON and OFF.|
|In this example the starting candidate is 6 in J4. The shorter blue chain simply says that when 6 in J4 is ON it removes the 6s in box 7 leaving 6 in G2 as the only remaining 6 and therefore the solution. On the other hand, the longer purple chain implies 6 cannot be the solution in G2. A 6 in J4 means 6s are removed from the rest of box 8 and that turns ON the six in H9. That forces 2 in J9 which in turn removes 2 from J1 making G2 2 and not 6.
Therefore the 6 in J4 cannot be a solution and it can be removed.
Nishio Forcing Chain example: Load Example or : From the Start