They also have a camera inside the liquid oxygen tank. You can see it drain away as the second stage engine runs... and then it cuts off. What do you think happens to the remaining fuel? What happens is that it's awesome.

https://www.youtube.com/watch?v=u656se4e34M

And here's extended video of the last section (I think from a different launch):

https://www.youtube.com/watch?v=8HZrrHI34x4