Well, lets look at the science of canning. When you put anything in a jar and heat it, it will expand. You want as much air to escape from the jar as possible. Therefore, you would just place the lid on top with no ring. Then, the air can freely escape. As the jar cools the lid prevents air from getting back into the jar creating a partial vacuum. The problem with just setting the lids on top is that when the water boils the lids will float away. So, the ring is placed on just tight enough to hold the lid in place. But, not tight enough to keep the heated air from getting out. When the jars cool the lids will pop in. Letting you know that they are properly sealed. If you wanted to you could crank the lid on tight once they have cooled. Some people remove the ring altogether. Since the vacuum holds the lid in place. Other people leave the rings on tight to keep it safe in case it gets dropped on the lid. Generally, you should heat everything you can in a pressure cooker.