The example used for testing is a game where you eat sugary things until you lose all your enamel (effectively making enamel HP)! Lillian shows how the aforementioned is tested. The TDD part comes in when someone asks to show a Test Driven Development example where brushing your teeth restores enamel.
It’s about 20 minutes long, includes questions/comments from the crowd.
Here’s the repo being talked about in the video: https://github.com/LeadPages/tutorials