A bow in the center (where the fold is) means that the fold is not a perfect 90 degrees from the cutting blades. The same is true with the June Taylor Shape Cut.
You can test this with a piece of paper and a rotary cutter. Fold the paper. Cut a strip in a way that you know is wrong -- with the fold not lined up perfectly perpendicular to the cuts. When you unfold the paper, you will see the bow. The more out-of-true from 90 degrees the cut is, the bigger the bow. If you cut at a perfect right angle, when you unfold the paper you will have a straight strip.