Helps to overcome the added junction resistance caused by heat.
How technical do you need it? Its been 20 years since school and classes in chip design. But basically think of the junction as a switch, but its a switch that is turned on by a resistor/capacitor charge circuit. If the junction resistance increases, the time to charge the capacitor increases. But your changing the frequency of the signal (overclocking so the length of the cycle is less), so the timing can be such that the circuit never actually gets to charge all the way (thereby not passing the right signal). Increasing the vcore, increases the slope of the ramp created by the resistor charge circuit. Meaning that the trigger voltage (which pretty much doesn't change) is reached faster so it effectively passes the correct signal.
Also the reverse happens. That same capacitor when asked to return to ground condition has to do so through the resistance of the junction. If it don't make it to the low trigger by the next rising or falling edge then a timing error occurs.
I know this isn't exactly what is going on, but may give some idea.
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