The secret Belgian binding is a method of bookbinding

Bookbinding

Bookbinding is the process of physically assembling a book from a number of folded or unfolded sheets of paper or other material. It usually involves attaching covers to the resulting text-block.-Origins of the book:...

that uses an exposed sewing to bind a textblock to the cover boards and spine. The cover is made using three separate pieces: front cover, back cover, and spine piece. The covers are attached by sewing through holes on the cover boards and passing over and under the spine piece. The previously sewn textblock is attached when the threads that pass under the spine piece also pass through the stitches on the spine of the textblock. The spine piece is held in place only by these threads passing over and under it. It creates an interesting thread pattern on the cover and it is a very sturdy binding that allows the book to open completely flat.
This binding was invented in the mid 1980s by Anne Goy, a Belgian bookbinder. She was looking for a Western version of the traditional Japanese stab binding techniquees. She wanted a book that would open flat but with the appearance of the stab sewing. Anne Goy calls this binding the "criss cross binding".