When you increase the voltage into a computer's CPU it increases the clock speed of the processor and can lead to better performance. Why does the increased voltage cause the processor's clock speed to change?
A CPU is, at its root, an extremely large sequence of FETs driving each other in complex combinations. These FETs have a very large number of possible states, only some of which are valid. Each clock cycle, the processor should move from one valid state to the correct succeeding valid state. The FETs are arranged such that they begin to switch from one state to the next only on a clock edge. But if one FET drives another, that means the driving FET has to complete its transition before the load FET can even start!
This is propagation delay, and the longest chain of propagation delays in the system determines your maximum clock frequency. You have to be sure that by the time the next clock edge arrives, all FETs everywhere have completed whatever transitions are going to occur. Otherwise you end up with your chip in an invalid or incorrect state, and all bets for correct operation are off.
So the minimum clock period (and thus maximum frequency) is a function of the longest propagation path in the chip, and of how long the individual transistors in that path take to switch. You can't change the propagation path once the die is etched, but you can change the time it takes for a transistor to switch. As with any FET, the switching time is affected directly by the time it takes the gate capacitance to charge. Since the capacitance is fixed, a higher voltage through the same FET results in more current flow, and a faster rate of charge. So increasing the rail voltage of the processor can increase the switching speed.
Of course, the down-side is that at higher voltages, the switching loss of the FETs also goes up, more than the gains from faster switching times make it go down. So higher voltage results in increased operating temperature, which can also affect the switching time of the FETs, and ultimately result in damage to the chip.
Without going in to details when you design a chip you spend a lot of time doing timing analysis between all the little logical and signal paths to each cell inside the chip. You can only run a chip as fast as it will meet this timing without getting errors.
This max speed is also affected by: process, temperature, and voltage. So you can either by design or by trying to over clock increase the voltage to increase performance.
Raising the core voltage raises the voltage to each cell of the chip and this decreases the prop delay through that cell in effect making the paths faster. You can then increase the clock rate separately to see if your chip still functions error free at this speed.
Of course you can break timing this way too or break your chip by damaging it with over voltage or over temp.
Your process and temp will affect things too so you could get lucky and get a chip that can already run faster than it's clocked, or unlucky and get one already running at near max speed.
In the before time people would cool down chips too to get them to run faster but now at the smaller process nodes we have temperature inversion so they run faster hotter.