This possibility, mentioned in passing, was used a lot in the age of processors with doubled clock speed (i.e. Intel SX and DX processors), now in disuse as it is not particularly advisable, and indeed quite dangerous for the processor.
Some boards have a voltage regulation of 2V to 4V (both for the jumpers and for BIOS). The voltage at which the processors work varies according to the make and model, but they range from 2V to 3.3V (except for the Athlons, which work at 1.6V). This means that by raising the voltage gradually you increase the speed (never increase the voltage more than 0.2V). However, we advise against this method as it overstretches two different areas of the computer: voltage and frequency. Each of these factors separately increases the heat produced by the chip, and the sum of both of them could lead to its deterioration.
NB: Overclocking is a technique which requires time to set up. It would be absolutely ridiculous to try and make a 500 Mhz processor work at 800 Mhz, so you should not try out the speed of the motherboard bus or the multiplier at speeds you know beforehand will not work.
To overclock safely and reliably, you should increase the values of the parameters gradually and cautiously, ensuring that both the memory and the PCI devices can withstand such speeds.
1 What is overclocking?