These are the notes on the web version of the Chinese course.
"Sometimes Chinese tones change based on the tone that follows it. There are three main places where you will see this: with all 3rd tones (low then rising tones), with the character 一 yī (1/one), and with the character 不 bù (no, not). Here we’ll take a quick look at how the 3rd tone changes.
There are two main rules for how to pronounce the 3rd tone: When you have two 3rd tones together, the first one becomes a 2nd tone (rising tone). When you have a 3rd tone followed by any other non-3rd tone syllable, it only lowers, it doesn’t rise at the end."