What was the purpose of the turbo button?

From Wikipedia's turbo button article:

The button was generally present on older systems, and was designed to allow the user to play older games that depended on processor speed for their timing.

Older games often would run programs as fast as the processor allowed. Since the developer designed the game for a 33MHz processor, as long as the user had a 33MHz processor, everything functioned as designed. Once the user upgraded to a 66MHz processor, though, the game now ran twice as fast, making it unplayable. The turbo button would slow the computer down to deal with this effect.

It would switch the processor between 4.77Mhz (the stock speed for an IBM XT) and whatever faster speed the processor was capable of - often 8Mhz. My hunch is that it was switching between two different crystals on the motherboard, but I don't have an old XT mobo lying around to follow the traces to be sure.