Change a file anywhere and every device updates itself. A deletion travels just as faithfully as an edit, which is why sync on its own is not a backup.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
two notebooks that rewrite themselves to match.
For example
You delete an old file on the laptop. That evening it is gone from your phone as well. You only deleted it once, but it left everywhere.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.