Draw on screen border in Commodore 64

Question

I have this curiosity for 25 years and I would love to understand the trick.

In the Commodore 64 the border was not addressable by the 6569 VIC. All you could do was to

Answer 1

Graphics in border: Either sprites OR using the $3FFF effect (which is actually NOT sprites at all). To go into in detail needs much more space and time than I have available here.