A firemaking bow is a heat producing lighting tool. Heat producing tools do nothing more than adding heat to an object, rising their temperature as consequence. If objects reach their self light temperature, they will light. In combination with a firestarter, they are very effective to create fire, as firestarters try to ignite resources.
Firemaking bows can be created in real life too. The rotational motion of the spindle generates friction to an objects, which generates heat as result.