The Test/Code Cycle
Specifically, it shows how unit tests and simple design work together while programming. Observe how the coding process occurs in small steps, just enough implementation to make each test run. There's a rhythm to it, like a pendulum of a clock: test a little, code a little, test a little, code a little.
I'm a big fan of Test Driven Development (TDD). My favorite aspect is that it allows me to write the least amount of code possible. I spent time making tests pass, instead of coming up with the coolest way to do it.