Actually, the electrons never speed up or slow down. They always move at the speed of light between atoms. The heat raises resistance because atoms get further apart, and the higher the resistance gets, the more heat is generated. At the point where the power becomes too much for the material to disipate, temperatures spike and the material is destroyed (maximum wattage).
It's not really that the processor is so hot that it stops functioning; rather, that the transistors can't switch fast enough to maintain a good signal strength. Operation becomes unstable because the signal becomes unclear.
(If you've not used an oscilloscope, this will probably not make any sense, but here you go) In a digital circuit, you need a uniform squarewave. Where there is the degradation of the signal from runing the cpu too fast, you can counter it by oversaturating the signal. So long as you can keep it cool, you can do so safely. (since upping the voltage will obviously raise the temp by creating more current flow)
Just incase: I wasn't trying to "flame" auld with that. I'm sorry if I came across that way...
What does Heat do to a CPU?
as zogmool says,increasing the core voltage speeds up the movement of electrons, - and this actually aggravates the problem,not alleviate it. You speed up your CPU,but generate more heat. Because heat itself speeds up electrons (generating even more heat),trouble comes when things get out of hand,resulting in "thermal runaway" - at some point the currents start to increase massively and blow up the chip's microscopic circuitry. When you "overclock",you need to compensate for the increased heat generated by improving the heatsink to maintain temperatures at a "reasonable" level- well below the "trigger" point. 
LOL i'm with you,GNXtreme - aint edukashun wunnerful?
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