Modern-day pressure cookers have built-in features designed to allow cooks to safely leave the appliance unattended during cooking times. Checking the appliance before cooking, thoroughly cleaning afterwards and using it according to the manufacturer's directions ensures safe operation.
The lid seal and what foods are being cooked are crucial in safe pressure cooking. The rubber ring that lines the lid is the rubber gasket. If it is cracked or dried out, replace before using. Furthermore, lids have locks that have to be activated before pressure builds. These same lid locks won't open until the pressure is released.
Filling the pressure cooker to only two-thirds full avoids the possibility of food blocking the vents. Specific foods that swell when cooked, such as grains and beans, or foods that froth, such as pasta and applesauce, may require only half filling the cooker.