VEVOR Media Storage Cabinet, 8-layer DVD Shelf Holds up to 464 CDs, Adjustable CD Shelves 16 Compartments, Protects & Organizes Music, Movie, Video Games or Memorabilia Collections, White

Package "C" has a plastic wedge that is screwed into the back of the storage for stability. There are no written instructions only small pictures that are not clear nor is this part of construction show in the video. How is this wedge placed that the screws have good purchase in the wood frame???