Secondary burn (secondary air)
Secondary burn is the re-ignition of smoke inside the firebox using preheated air injected through tubes or a baffle above the fire. It is how non-catalytic EPA-certified stoves achieve low emissions — the visible "light show" of flames rolling along the top of the firebox.
Smoke is unburned fuel. A secondary-burn stove insulates the firebox to keep temperatures high and feeds hot air above the wood so those gases combust before entering the flue, delivering more heat per log and far less creosote.
Secondary combustion needs a hot firebox to work, which is why non-catalytic stoves burn cleanest at medium-to-high output and why an extended choked-down smolder defeats the design.