 At this stage, the heat source is reduced to the lowest possible heat that still maintains pressure, as extra heat wastes energy and increases liquid loss.Before the pressure cooker lid is sealed airtight, the internal air has to be mostly replaced by steam.Steam has a much higher specific heat than air, and the presence of steam rather than air inside the pressure cooker is how it is able to transfer sufficient heat into the parts of the food that are not submerged in liquid, such as a pot roast.