Undervolting your CPU is completely safe. There is no physical/electrical mechanism associated with undervolting that can cause damage to your CPU. However, there are some things you should know and be aware of when you are undervolting your CPU.
When CPUs are manufactured, there are variations in the manufacturing process that produces CPUs that can achieve different maximum clock frequencies… i.e.. some are faster and some are slower than others. The manufacturer tests these CPUs…. finds which ones are faster and which ones are slower… labels them and sells them at different prices… even though they may have all been produced exactly the same. However, when they say that a CPU will work at a certain frequency, say 2 Ghz, this really means that they have tested that the chip will work at this frequency plus some margin of error (say 10% or 2.2 Ghz (I cannot say what margin of error Intel uses in its manufacturing test)) at the worst case operating voltage and the maximum operating temperature.
It is important to note that worst case operating voltage is the LOWEST voltage that the processor is speced to operate at as in general, the lower the voltage…. the lower the operating frequency. Additionally, note that at higher operating temperatures, the slower the CPU runs. CPUs when first turned on typically operate at room temperature (or about 25 degrees C) but are typically speced to run at a maximum operating temperature of 85 degrees C (or sometimes hotter). If you could somehow keep the processor closer to 25 degrees C, it would certainly run at a higher clock frequency than if it heats up to 85 degrees C.
So what happens if you undervolt the CPU. First, you are reducing the maximum operating frequency of the processor. Depending on the processor and the voltage change, for example, you might change your 2 Ghz processor to a 1.8 Ghz processor. However, remember that the manufacturer really tested this processor to work at say 2.2 Ghz, having some margin of error. So, your 1.8 Ghz under voltage processor probably is still a 2 GHz processor… but with little margin of error. The only way to know how low you can lower the voltage and test to see if the computer keeps working.
So what is the best way to test how low you can lower the voltage to see if the computer is still working? One obvious test which is a very BAD test it to simply boot the computer. Of course if you lower the voltage and you cannot boot the computer than you have definitely lowered the voltage too much. But if you CAN boot the computer, this DOES NOT mean that you will have a functioning computer. Why? Because when you boot the computer, your processor is probably running at a cool 25 degrees C and is therefore running in its FASTEST thermal operating zone. So, in order to know if your computer will be reliable at the under voltage, you have to have the computer running under some more extreme conditions, where the processor is working at its hardest and generating the maximum amount of heat… and for a sufficiently long time to get the processor up to its maximum operating temperature… around 85 degrees C. If the processor still runs at this temperature than you have found a safe under voltage to us. Some applications that might do this are video compression, 3d rendering…
There is an additional complexity that you may also have to consider depending on the processor and laptop you are using and the options available in the Intel’s Extreme Tuning kit. One feature that some laptops use is a temperature sensor on the CPU and an automatic throttle back in the CPU clock frequency when the processor starts to get too hot… thus allowing it to cool down a bit before it brings the clock frequency back up. This obviously adds some complexity, but may also help your processor to stay running at the lower voltage by not allowing the processor to heat up so much. You may also be able to lower the temperature at which the system throttles down the clock frequency to allow your system to run at even lower voltage. This will of course reduce performance in the cases where the clock frequency is throttled back… but depending on you use scenarios, you may not care about this.
The other feature available in CPUs is a turbo boost mode. Given that most laptops are running applications like Word, etc. that do not make heavy use of the processor, they typically do not heat the processor up too much. And therefore, when the CPU is cooler, it can run at a much higher clock frequency… but only for a short amount of time… i.e. until it heats up to the point where it can no longer run at that frequency. If your processor supports such a turbo boost mode, it may or may not work at the lower voltage you selected and you will probably want to run a test to determine if your processor’s turbo boost mode still works.
The advantage of undervolting your CPU is that you can extend the lifetime of your CPU… first by reducing the temperature that the processor runs at on average and thus the thermal cycling failure mechanisms. In addition you will reduce slightly the hot electron injection into the gate electrode failure mechanisms which are reduced with lower voltage. However, processors usually last plenty long today… usually longer than the amount of time most people want to keep their computers before they are longing for a much faster and newer model so I am not sure this enhanced reliability is anything of value. And finally, it is worth noting that even though operating at lower voltage does somewhat reduce the maximum clock frequency, it does at the same time reduce the power dissipation and therefore also the operating temperature of the processor which tends to increase the maximum clock frequency… which depending on application usage, can somewhat balance each other out. And finally, the under voltage will reduce power consumption and thus enhance battery life, which maybe is your main reason for wanting to under volt your CPU in the first place.
