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

From what I understand, this is only possible for sprites.

Sprites are allowed to be in the border area, and the border gets drawn overtop. Through some trickery, the border can be removed.