The smallest box and tower containes the combustion chamber. Wood is fed vertically into the black metal opening in the shortest box. The wood then burns sideways and then up the short tower. The combustion path is highly insulated to facilitate temperatures in excess of 1800F. This insures that all smoke, creosote and other volatiles are consumed. You can read more about the combustion system here…
At the top of this tower, the super hot exhaust is then routed into the primary bell (tallest of the towers) where the hot gases rise. The heat is absorbed by the tower's masonry, the exhaust gases cool, become heavier, and thus sink and exit at the bottom. Depending on the design, the exhaust can enter another smaller bell (tower) where the same process happens again, or in the case of a single bell, the exhaust exits to the chimney.
This design approach for heat capture facilitates a very strong draft, since there is no friction from flues. It provides for smaller foot print designs since the heat capture is vertical rather than horizontal and it is inherently more efficient than flues, meaning you need less mass to capture the same number of BTUs. You can read more about why bells are more effective here.
By using easy and low cost construction materials masonry style heaters can now be built for substantially less than traditional masonry heaters.