Running in Circles is Basecamp’s view of agile product management. They acknowledge the value of working in cycles, but add three pieces: having the time to focus, being able to modify the original plan, and tackle the core unknowns of the feature first.
The first two are enablers that are provided to the makers by management. The last part is how the maker make the most of those powers. Together, they form a process that is nicely captured with the uphill / downhill metaphor. Uphill you are discovering the unknowns and making decisions about what goes in, downhill everything is clear and you are implementing it at warp factor 10: