The skin is gelatinous. Lots of gelatin comes from skin. Roll the skin and tie it, like this. Braise it in anything, and it gives body. Keller points out several times as he goes along that the recipe utilizes everything.
Nothing is wasted.
Additionally, this process creates viscosity, instead of the old method of using flour. Finally, he ties the skin up tightly, seasons it with salt and freshly ground pepper, and begins the cooking process. Before refrigerating for the night, Keller will sprinkle some pink nitrate on the meat. The nitrate is pink so that it is not mistaken for salt. The nitrate is a cure, and prevents botulism.