Heat is transferred from the air and cooker to the food by conduction, convection, and radiation. As the cooker heats up itself, it transfers heat from its quickly heating up metal (low specific heat) to the food through conduction. As the high pressure gas moves around the food, the food is also cooked through convection. There is probably some radiation cooking the food also as electricity is required to run the cooker. As we opened up the pressure cooker, large amounts of steam rapidly bursted throughout the room as the air moved from an area of high pressure to low pressure. We needed to be careful not to stand directly over the cooker as we released this pressure so that we were not burned by convection. The resulting stew was delicious as we understood the physics behind cooking it.