Explain to me how this works

[bububassboner]

So on youtube I saw a video of a guy who made his own tube drawing bench and draws out a trumpet mouthpipe. I see what is going on but I really don't understand how it works. Could someone explain how this works? From what I read, this process ends up making the tube longer and it reduces the I.D. and wall thickness but I don't get how. For reference here is the video I saw https://www.youtube.com/watch?v=OffF5BuGaFA" target="_blank

Thank you in advance.