Have you ever had a top-filled deer feeder?? or a top filled Cat feeder?? even some bird feeders work on the same principle.. the feed has to be eaten from the bottom, which makes room for the stored feed above to drop down into the feed pan.
The fire is built on the grates, and as the coal fire gets fully established, more coal is added over the fire.. The hopper is a rectangular box with an open bottom and a sealed lid on the top. When you fill the hopper, the coal at the bottom is blocked by the coal on the fire, so it can't leave the bottom of the hopper untill the fire burns some of the coal to ash, the level of the coal drops, making room for some of the coal in the hopper to drop out of the bottom of the hopper onto the fire..
It is a gravity-feed stoker.. with standard shaker grates. Or it could be described as a hand feed stove with an extra deep coal capacity.