Swimbill-
I've seen a couple variations, but they work the same. The two long boards with the sand paper applied to the inside are mounted to the base, at the taper you are looking for. There is an arrow stop at the far end. Looks like the cleats on top are used to keep the shaft between the sanding boards. You chuck a parallel shaft in your drill, then use the drill to spin the shaft as you push it towards the arrow stop. When you get to the stop, you have your taper! Simple and brilliant!