I'm not sure I'd consider it a gap between student understanding and what a student can do because student understanding is in fact, what a student can do. When I assess a learner (i.e., student understandings), essentially, the entire process depends on what evidence the student is providing. I see formative assessment where there is less of a boundary between assessment and instruction as the avenue for providing the "learning ecosystem" needed in order for students to perform in a way that provides the evidence required to make reasonable inferences on student achievement. The following questions were posed:
What are the assessment strategies and tools that allow us to collect the most accurate picture of student understanding? Which methods of assessment actually widen the gap between student and teacher? Which come closest to allowing us to “sit beside” our students? Does any of this really matter when it comes to quality teaching and learning?
Formative assessment - as opposed to summative assessment - allows educators and students to "sit beside each other" as well as students themselves sitting beside each other in a more cooperative learning community. Specifically, formative assessment that is "baked in" to the following approaches are in order: questioning techniques (aka Socratic Method or instructional conversations), performance tasks, projects, and problem-based learning. These approaches, methods, whatever, provide higher-order thinking that is more likely linked to leading learners to think outside of the box and to be more creative in how they interact with people and materials (i.e., forming a socio-technical organization based on the principles of semiotics).
Having more formative assessment than summative (and we can have both) is the best way to provide the constant feedback loops necessary for all educational stakeholders to grow and learn from the educative experience that a classroom can provide. But I feel that teachers are in the best position to be advocates for working with all other educational stakeholders in making it all happen. This is only possible if teachers are free to take risks, make mistakes, and share their successes and failures through open and constructive discourse.