In shelf and hoof mushrooms, there is a thin layer between the hard outer shell and the "gills" that make up most of the rest of the mushroom. For traditional Amadou, that layer is cut out, lightly hammered to "fluff it up" a bit, then soaked in a strong solution containing potassium nitrate. That PN then gets into the fibers. When it then dries out, it will catch a spark fairly fast, and burn ... actively! The PN adds oxygen to the sparks, so it really burns well and is hard to put out. Great for fire starting - especially with some damp tinder.
The Vikings got their potassium nitrate by boiling down urine to concentrate it. Then putting their future "amadou" in it to boil for a while until thoroughly saturated.
Fly fishing sight? I'll have to go follow the link to see just what they are using it for.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands