If my memory serves me well this worked only with the PAL version of the C64. Because the video chip (VIC) was so tightly coupled with the main processor and RAM, the video refresh rate determined clock speed. The PAL C64 was clocked at 0.98 MHz, the NTSC version was 1.25 MHz, so US C64's were faster per se.

If you had a PAL version you could switch between both frequencies with a switch soldered to some place on the board (or maybe it was the user port), but of course you then lost video output as long as the switch was in the NTSC position.

EDIT: Wikipedia says the NTSC version was only 1.023 MHz, so I remembered that number incorrectly. Maybe you were referring to another trick that would give you a 10%-20% speed increase?

The VIC-II "steals" CPU cycles every 8th scan line in order to load sprite & color data, what C64 folks call "bad lines". It has a BA (Bus Available) pin that's fed into the address decoder PLA. When BA is low, the PLA tells the CPU that memory is only available for writing.

If you disable the VIC-II's output with a poke, it doesn't need any "bad lines" to fetch data. The CPU then gets 25 scan lines' worth of time to crunch data each screen refresh.