Commercial Vacuum Sealers are a whole nother ball game in preserving food for your family and careing for your fish and meat harvests.
This type of vacuum sealer uses an inclosed chamber you will place your bagged product into.
When the pump starts, and your lid closes a vacuum is created in the chamber. Because your bag is open and in the chamber, as the chamber pressure decreases, the air inside the bag flows out into the vacuum, when the chamber is out of air your bag is also out of air. The bag will look like it is puffing up or filling with air, it is in fact being held open by more pressure on the inside of the bag than on the outside of the bag.
Now the magic starts. When the vacuum leavel has been achieved the heat seal bar will lift up and press the open end of the bag against the lid get hot and seal the bag, now your bag is in a vacuum with a vacuum in it and around it. At this point a valve will open and let the air back into the chamber and your bag will collaps due to the added air pressure on the outside of the bag and presto your bag is vacuum sealed.