The paper presents the technical view of constraint mechanisms, especially movable polyhedra. Basic parts of this type of mechanism are flexible polyhedra with four faces. These, coupled at the corners of one main facet with different section geometry having k number of edges leads to a polyhedral with 2k+1 flexible facets. The special focus lies on the quadrangular section with 4 edges, which leads to a nine face polyhedra. The typical property of such mechanisms is the foldability, which offers a wide range in technical applications. The planar equivalent of these mechanisms is the parallel crank mechanism. The special properties of a parallel crank mechanism help to understand flexible polyhedra and also to reduce the analysis of some simple equations. Therefore, its inverse task i.e. the synthesis of flexible polyhedra is simple and can be given as a geometric algorithm. The focus of this paper aims at the technical application of flexible polyhedra. So, the synthesis of a nine face polyhedra is shown for a foldable ramp in a lightweight transportation system. This example helps to present the advantages in stability and construction.