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

You can open upper and lower borders with a simple BASIC program:

    1 poke56334,0:poke53266,212:poke53265,27:poke16383,0
    2 h=53265:i=53273:h1=19:h2=27:i1=1
    3 pokei,i1:waiti,i1:pokeh,h1:pokeh,h2:goto3