If you're unfamiliar with the ZPD, it's an interesting concept: it's a way to visualize the potential of a student/learner. It shows the growth, and limitation of growth. But once a learner has sound knowledge up to the ZPD limit, that now becomes the smaller "what I know" zone, and growth and understanding continue onward.
Vygotsky's Zone of Proximal Development by Dwayne Schnell is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.