A computer's BIOS can change either the multiplier value or the (FSB) clock frequency to overclock or underclock a CPU. While overclocking by changing the multiplier value is easier and in more often cases safer, it can only be done on CPUs with unlocked editable multiplier. To overclock/underclock a multiplier-locked CPU, you have to change it's frequency. Now I want to ask: what is this frequency(often called FSB frequency)? Is it the system frequency that is supplied by the system's oscillator crystal(i guess so, since it changes also the RAM frequency)? How does the BIOS change it? Is the frequency of the crystal somehow dependent on supplied voltage?
Many modern mainboards utilize a system of clock distribution based on clock syntesis. A quick example would be IDT/ICS chip like ICS954101 (although I am not sure if this particular IC can change FSB, but as an idea). The main reference crystal is typically 14.318MHz, the rest (CPU FSB, DIMM, PCIe, USB, SATA, etc) are derived via PLL and are programmabe over SMBus/I2C bus. That's how some mainboards may have some frequencies tunable from BIOS, all depending on how "liberal" their BIOS is.
Frequency of the system clocks is definitely independent of supplied voltage, the voltage on clock chip is well stabilized. But some board voltages may have an independent control via a different power management block, and the BIOS might have linked tables that allow higher clocks if higher voltage is set, for overclocking enthusiasts.