Final answer:
Back casts and reach casts are fly fishing techniques used to build momentum in the fly line for distance and to present the fly to fish, respectively.
Step-by-step explanation:
The fly activity mentioned in the question that includes back casts and reach casts is fly fishing. Back casting is a technique used by fly fishers to build momentum in the fly line to reach a desired distance or to reposition the line for a new cast. Reach casting allows an angler to present the fly to fish that are upstream or across the river from their position by extending the line outward in the direction of the target. These techniques are essential for successful fly fishing, aiding in precise fly placement and making it possible to fish in a variety of water conditions.