Nishio is a slightly controversial solving technique that examines the effects of placing a candidate in a cell on the remaining candidates for the same digit. When a contradiction is found, the tried candidate can be eliminated.
To avoid the smell of Trial & Error, players often perform Nishio moves in reverse. To understand how this works, you need to consider the nature of a Nishio contradiction. It occurs when all the candidates for a house are eliminated, as a result of which the house cannot be completed. In the reverse operation, all the candidates for the conflicting house are tried in turn. In each case, the original candidate will be eliminated. Because this proves that there exists no case in which the original candidate can exist, you may eliminate it from the grid.
There is a wide range of single digit solving techniques which diminish the need for Nishio.
Nishio can be replicated by computer programs using templates.