The idea is that rain flows downhill, you dig the half moon shape to capture the water on the end without a ditch and then it sinks into the ditch instead of flowing unobstructed to the river and taking all soil with it.

It's an ancient practice that was forgotten and rediscovered. The beauty of this approach is that it shows results within a few short years. Basically in Africa if there's water, nature shows up and consumes it. So you get lush growth and rapid soil restoration. Trees, vegetables, etc. on what was a heavily eroded flood plain before.

It's easy to explain, the locals get why it works. And they get a very fast response from nature and all the produce and riches that come with that. And all they need is shovels and some elbow grease.