Question about a solving technique

[Realshaggy]

Think of it as a shortcut to the solution which is unique, but you can't see how to prove it yet. For this, it has to be unique by other means and your example is not complex enough for that. Your "x" digits are not irrelevant, they are needed to "enforce" a unique solution from outside the marked cells.

Andrew Stuarts Sudoku solver usually has detailed examples on the description pages and a discussion section that might lead to other examples or sources.

https://www.sudokuwiki.org/BUG