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What is undervolting? You reduce the input voltage of the CPU to a working minimum, below the default manufacturer setting that is unaccurate. The CPU will still work 100% but it will draw less power, be cooler and more efficient.
The exact voltage value depends on your components and your CPU sample. You need to find out yourself what's working and stable/unstable.
Let's say you put a voltage offset of -0.050V. The CPU will be cooler and should still work without programs crashing etc. When you go lower it may result in instability: Bluescreens, crashes, freezes. While it sounds scary, but you must hit the instability to know how low your CPU can go.
That's why you need to know how to test against instability:
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What's the perfect voltage for me?
Nobody knows and there's no universal answer. If you experience instability after lowering the voltage, you must increase it by 1 or 2 steps and test again. You are doing a jeweller's job by exploring the limits of your hardware. This fine-tuning yields great results. Undervolting = cooler CPU, less power consumption, less noise. Overcloking = faster CPU, saved money (because you didn't buy the more expensive CPU) and fun.
PS: What to undervolt?
CPU Cores. GPU/Internal GPU. Possibly CPU Cache like in the video above. These are easy to test for stability. Never touch the I/O or something else you don't know. If you change the I/O voltage, you may break it so that corrupt data will be written to disk etc. You don't want that.
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