I don't know about the Yanomamo, but the practice is fairly common. The material I saw came from the pounded root of a Plant I never had an identification for. It was a vine. The pulped material was tossed into a very slow moving stream or a pond and the fish would surface down stream or be washed into a net set in place by the fishermen. I saw this done by Brazilians at one of the gold mining villages I visited and by native peoples I met in various areas.
Rotenone is available in some gardening stores as a natural insecticide.
We use a plant that is locally known as Wild Cucumber. The vine it kicks out has grape like leaves and round, spiney fruits. It is crushed, and the crushed material is tossed into a stream much as I described above. It seems to cause the fish to suffocate. If they aren't harvested, or wash out into faster water, they recover and swim away.