I have used this trick many times when gluing up a cabinet with a web frame towards the center of the piece. It’s difficult to apply pressure directly to the center frame because a typical clamp will not reach. However, by placing a thin strip of scrap inside of a long caul (I use a Popsicle® stick) as shown, and clamping the ends with bar clamps, direct pressure is applied to the center. Another way, of course, is to use a softwood caul with a slight crown.
– Joel Gaertner