Ah now I'm getting the idea, the burn chamber is a lot larger then the air intake and port towards the riser, that way the burn material does not have to be tighly packed like a sawdust rocket.
Just stuff the burn chamber and the restrictions at both points keep the burn under controll while the door keeps everything in the burn chamber and still alowing the riser to complete the secondary burn unlike a system with flu pipe restriction.
It makes sence when you see it, almost makes you think "why did I not think of that"
I can see how you can burn any lose material in that: just let it dry, not even chopping up is needed.