The joint in a glued up panel dries nearest the ends first. Many woodworkers have a tendency to run the stock over the jointer carelessly. This sometimes causes the boards to have a snip (the first and last inch or two of the board has about 1/32" more material removed from the jointed edges) on their ends. If these 'sniped' boards are glued up into panels, eventually the boards dry out and the joints start to come apart. Springing the joint helps eliminate failure of the joint. Using a plane such as the Lie-Nielsen No. 9 Edge Plane, plane the edges to where you have a gap of about one fine shaving running all but the first and last three or four inches of the joint. This gap is sprung or closed under clamping pressure and as the ends of the board dry and shrink, they have less tendency to come apart at the glue joints.