The basic idea is if you have something looks this:

- Code: Select all
`+---+---`

|437|596

|852|317 ...

|961|824

+---+---

The 2's and 1's can be switched and the result is still a valid sudoku solution regardless of the rest of the puzzle. Therefore at least one of those four spots must be a given or there would be more than 1 solution.

You can take advantage of this when solving a puzzle. Take this example:

- Code: Select all
`+---+---`

|...|...

|..a|.b. ...

|..c|.d.

+---+--

In this example lets assume you deduced:

a is either 1,2, or 5

b is either 1 or 2

c is either 1 or 2

d is either 1 or 2

Then a has to be 5 because if it weren't then the result imply that the puzzle has multiple solutions.

I am sure that there are plenty of different circumstances besides this where you can take advantage of the knowledge that the puzzle is valid. Not sure how often they occur in real puzzles, but I thought I would throw this out there and see if anyone knew of it already or finds it useful.