It could be that a tight fitting velvet cover provides the necessary heat sink for cooking, as well as offers some insulation to the cook pot itself to help retain some of the heat. With the cover in place, heat is conducted directly to the pot by touching, which is probably why a loose cover does not work as well. And with the cover, a portion of the heat is retained from being reradiated away from the pot, like with a simple black pot. Usually this lost heat is captured by the air in the box cooker and redirected back at the pot in a convective heat transfer fashion. Retaining the heat is more efficient than first losing it, and then recapturing it. A heavily insulated jacket may have diminishing desirable results however. The amount of heat retained or lost needs to be balanced with pra