The cereal/decoction mash is in a pot, that goes into the pressure cooker. The pressure cooker has a false bottom and water under the false bottom. The goods never go over the steam temp, so no scorching, and no need to stir.
I'm sure we all know this, but I think it's worth pointing out that the steam is under pressure and will be hotter than 212F. How hot depends on the pressure. Just to be clear.