Throw-up is the degree to which the spine flexes in the opposite direction from its usual shape, or rises off the table if the book lying flat. Drape is the degree that the leaves flex. For example, a thick, small, cross grain leaf has virtually no drape, while the same paper might drape very well in a larger format when bending with the grain. Tom Conroy’s The Movement of the Book Spine discusses and illustrates these differences very well.
The above tight back book, which I rebound, was resewn onto 5 cords, laced into handmade pasteboards, and covered in calf. It exhibits high throw up and well draping leaves. In this case, the sewing, sewing structure and spine linings were carefully chosen to achieve what I consider an optimal opening. This book lies flat without strain, yet when closed there is not excessive torquing or text-block drag
Rebinding is a major, very invasive, very expensive treatment, and rarely necessary. But it is hard to deny the appeal of a new binding: well functioning, easy to handle, and you do not need cradles to consult or read it. Of course, what is lost is the context of the text, the authenticity of the book in its totality, and simply the appeal of an antique object.