Why is quick sort called a tail recursive algorithm?

I know what a tail recursive algorithm is as written out in this SO answer. However I am going through this video of quick sort algorithm from MIT and at 18:30 seconds the professor says that this is tail recursive algorithm. I fail to connect how this is tail recursive . We are not doing calculation at any step of recursion or are we ? Can you explain why this is cited as an example of tail recursive algorithm . Please base your answer on the premise that I know what an recursive algorithm is . The part that isn't clear to me is why it is called tail recursive ?