These line markers from Japan are really useful for tenkara, microfishing and even nymphing with a fixed line.
You run your line through the holes on the marker. The idea is to keep the marker on your line but out of the water. As you drift your nymph or hook with the current you can then watch the marker instead of trying to keep an eye on your line.
The marker will indicate even the most subtle take from a fish. It will also help you learn the flow of the water.