Teaching TDD... but what just happened?

In my CS222 class, I spend a lot of time talking about, demonstrating, and giving feedback on Test-Driven Development (TDD). Indeed, from the very first day of class, we're doing TDD. For several years, I've opened the course by talking about confidence and how we can take small steps to maintain it. Last week was the fifth week of the semester, and my students had just started the "two-week project." That project integrates the concepts from the first four weeks of the semester into a single deliverable that of course includes TDD.

On Thursday, I was showing them how we can write a custom parser for data pulled from Wikipedia. I started by writing a test that, given my sample data, my as-of-yet unwritten parser could extract the piece that I want. Then, we wrote a clearly naive implementation of the parser that just returned that value as a literal. The literal has no place in the specification, and so our refactoring moved us to a correct solution. I made a big deal about how we had just completed a red-green-refactor sequence, summarizing each step to them as I have just done for you above. I especially emphasized that at the time of writing the test, we didn't care at all how we would make it work.

A student pushed back a little on the solution, asking why we didn't just go back and change the test case itself to something else. I responded that this would not be right, because before we wrote any production code, we first had agreed that this case was correct--and it was. I reminded them again about red-green-refactor. At this, a different student raised his hand to comment. These are not his exact words, but his observation was essentially this: "Are you saying, then, that at the red step, we write a test that tells us what we need next, but we don't worry about how it will work; then we make it work however we can; and then we clean it up to make it work right?"

I paused for a moment and then agreed, telling him that I really couldn't have said it better myself, thinking to myself as I said this that I had, indeed, said basically that same thing, many times in the past five weeks. At this, several heads in the class nodded sagely, making expressions as if they had gained some new insight into the process.

On one hand, I am happy that they seem to understand it now better than they did before, that this students' summarizing the process seemed to resonate. Yet I am plagued by the question, What just happened? What was actually different about this explanation than before?

I will see the students again tomorrow, and I'm thinking about just asking them. I don't want to make anyone feel badly about not understanding it before, but I do want to know if they can identify what tipped them toward understanding it now.