>>29034465
It's simpler than anyone's seeing. The zip-up doesn't have enough space for her boobs, but is otherwise large enough for her frame. When it's zipped this causes part of the fabric to have to stretch, making a band of fibers that are oriented differently than the surrounding landscape. If you pay attention to the clip you can see that this band of stretched fibers cuts across her breasts horizontally. This indentation makes it look like her breasts stop where it begins, it's a false underboob gap. When the zipper is released so is the pressure creating that indentation and voila.
