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

Im going to take a stab in the dark here (havent done this myself). I found this on wikipedia.

The c64demo section explains that:

Effects thought impossible were achieved in demos, mostly due to undocumented side-effects pertaining to the MOS Technology VIC-II chip. Some examples for VIC-trickery:

One of the mentioned hacks was:

Sprite scrollers were placed in the border. By tricking the hardware not to draw the border around the screen, sprites could be moved into this area and displayed.

Of course, the interesting part of the question is how its done. I would suggest looking in some demo database which includes source code in search for a demo that employs the hack.