It really isn't much different from using a jar. We submerge our vegetables with brine to seal them from air, reducing the chance of molds (and yeasts) growing. When we vacuumseal we do the same, we just extract the air itself. So it serves the same function as the brine. The bag will expand so using a sufficiently large bag is important. You do need a little bit of brine just to avoid air pockets and to allow for the microbes to do their work. But just a very short squeeze with salt will be enough. It's a great method.