For better results, make sure:

• You keep black edges around the book.
• You keep up and down edges symmetrical.

Finger hiding: Up and down edges are symmetrical

• The document has a margin in which your fingers can fit. Indeed, without margin, the result of the option 'Finger hiding' cannot be guaranteed, because the fingers that overlap text or image are notdetected.
• To avoid pages with colored drawings.

Finger hiding: avoid pages with colored drawings