Lock the lid in place, bring the cooker to full pressure, and cook for about 30 minutes. Turn the cooker off and let it cool and naturally release its pressure, about another 30 minutes. When it's totally depressurized, carefully open the lid and test the ribs. They're done if the meat falls off the bones. If they're still a bit tough, close up the cooker and bring back to pressure for another 5 minutes, then let naturally cool again. When ribs are fully cooked, remove them carefully from the pot, place on a platter, and tent with foil to keep them warm.