Comment by gravenimage

Using the 'secret' is a common way of deriving further digits in a Sudoku - if you know the sum of digits in a row or a box has to be X, then the missing digits must sum to 45-X. In the 'Cracking The Cryptic' videos, Simon is always careful to explain this, and always prefaces it with a warning that he only tells it to his closest friends (all 400k of them).