The traditional method is to use two lengths (fabric the length of the quilt). Split one in half lengthwise, then sew a half to each side of the uncut length. This results in the back having two seams. Traditionally a single seam down the middle has been avoided.
The Flynn method shows how to achieve a similar result with less than 2 lengths of fabric (saves on fabric cost). To be honest, I just can't seem to stay with his explanation long enough to actually use it.