Dr. David T Conley, writing for Education Week:
A classroom well balanced between breadth and depth might introduce new concepts on a regular basis and practice them to ensure basic understanding while at the same time have students always working on one project or task that goes deeper in a keystone area. While the majority of class might still be used to introduce, explain, and practice new content, a significant portion of class time might be devoted to projects and tasks focused on keystone concepts, which students would spend considerable out-of-class time on as well.
The key word above is "balanced."