How does the Wonderbag work?

The Wonderbag is a non-electric slow cooker that uses heat-retention technology to continue cooking food once your pan has been removed from the heat source.

To cook, simply bring the food to the boil on a stove, let it simmer, and then put the pot into the Wonderbag. Cinch the Wonderbag tightly with the drawstring, and allow the food to slow cook to perfection for up to 8 hours.

It works by retaining the heat within the pan, and maintaining this heat for several hours, helping to continue cooking the food without additional energy. 

See here for more details: https://wonderbagworld.com/pages/how-it-works