When pressure canning, a critical step to achieving proper pressure inside the canner is allowing it to vent steam. What does this mean?
After placing jars inside the warm canner and attaching the canner lid, set the stove burner on high. Watch for steam to escape from the vent pipe. It should be a strong blast of steam that is visible in a funnel-shape. Let it continuously steam in this manner for 10 minutes. This allows the steam to build inside the canner and push the air out so that the canner will build pressure properly. After the 10 minutes, add the weight or counterweight to build pressure inside the canner.
Air trapped inside a pressure canner lowers the pressure achieved, but more importantly, it lowers the temperature obtained and will result in underprocessing and unsafe canned food.
Source: National Center for Home Food Preservation and USDA Complete Guide to Home Canning
