Generally speaking, a stove has three mouths. One is a big mouth at the bottom, which is used to scoop out ashes and feed air. There are two mouths on it, the big one is used for adding coal, chopping wood and cooking, and the small one is an air duct, where cigarettes are taken. When you cover the big mouth above, the air duct, the combustion chamber and the air inlet below form a pipeline. When the hot air in the furnace is exhausted through the air duct, fresh air will enter from the lower opening and the air will flow, just like an exhaust fan. The fuel in the combustion chamber will burn more and more because there is a lot of oxygen.
The same is true of earthen stoves.