Can someone please “explain this to me like I’m five”:

https://www.6809.org.uk/xroar/online/?-run=512scroll.cas

Click that link to see it run in the web browser. This screen shot does not really show how it looks running:

It shows a CoCo 32×16 text screen smoothy scrolling semigraphics blocks and text characters up the screen. The note that the SAM is not involved is also something I need a child-like explanation of. What does the SAM actually do that would be part of such a technique?