Wrinkles and folds are a usual damage in documents made of paper, cloth or parchment. The common way to restablish these supports to its original condition is by pressure: under weights or in a press.
When the paper has a relief which must be preserved, it is then not possible to use pressure, as it would smash this relief.
Flattening under tension may be an interesting alternative on certain occasions or when a selective flattened is needed. It takes advantage of the natural shrinkage of the paper when going from wet to dry, so that the tension is given by the same paper.