Basically you write a test first, say it calls an undefined method `x.somemethod()`. You push a button to run the test, as soon as the method call fails, the environment shows you a popup which lets you define the method right then and there, while you have all the details of the suspended execution. You can write the method and the 'resume' the execution from where it excepted.

You first extend the environment with tests, then add the code to make them green. All without ever having to restart the "program".

http://files.pharo.org/books-pdfs/updated-pharo-by-example/2...

There are pictures :-)

There were some utilities that let you run the tests at the push of a button.