In Agile, The Product Owner starts the iteration showcasing the work of the previous efforts, and then focuses on preparation and planning for the coming iteration.
The Development Team first learns and inspects the project requirements, while reaching their highest productivity in the middle of the iteration. The Scrum Masters job is to make sure this productivity is not interrupted. Exceptions can be made, but will negatively effect productivity and the planned delivery of value to customers.
The graph below illustrates how the Product Owner at the start of the iteration is putting in higher effort for planning, to have the development team be the most productive during the middle of the iteration. This is the cycle of an iteration.
The graph below illustrates how the ebb and flow of Product Owner and Development Team.