I've figured it out in my head but haven't tried it. Try it with a piece of paper. Cut it on the diagonal. Keep the two pieces side by side and slide one side up and the other down. You'll see how it works. The more you slide the pieces the wider the fabric gets. There will be triangular points at the top and bottom to be trimmmed off in order to make a rectangle.
I haven't done it yet cause I don't know where I can lay out my fabric so I can cut a straight diagonal. Or how to mark and cut the diagonal for that matter.