Normally (if load-line calibration is disabled) you'll notice a significant drop in CPU voltage when the processor goes from an idle state to a load state.
This voltage drop is called vdroop and can be seen in the picture below (which is from an Anandtech article on the subject). The voltage was set to 1.25 V in the BIOS (shown as CPU VID in the picture), and as you can see, this voltage threshold is never exceeded.
CPU Voltage with Vdroop
If you're looking for vdroop on your own system, you'll only see it if all of the power saving features (such as SpeedStep, C1E, and C-States) are disabled. Vdroop is still there even if the power saving features are enabled, but you just won't be able to notice it as easily.
Vdroop is designed to help
To most people, this voltage drop looks pretty bad, but vdroop is actually a built in feature by Intel and is designed to make sure the voltage level never goes over what you set in the BIOS.
This is necessary because there is always an overshoot whenever there's a voltage change, and without vdroop, the overshoot could make the voltage higher than what you set in the BIOS (for a split second), which may damage the CPU.
This can easily been seen in the picture below, which shows the voltage go as high as 1.29 V when vdroop is disabled. However, this is a problem because the CPU voltage was only set to 1.25 V in the BIOS.
CPU Voltage without Vdroop
If you compare the load changes as shown in the previous two graphs (shown as Light to Heavy Load Change and Heavy to Light Load Change), you'll notice that it also takes longer for the system to stabilize after the load change when vdroop is disabled. This is because it's harder for the voltage regulators to maintain the constant voltage all of the time.
How vdroop got its bad name
Vdroop got a bad name when some older, low-end motherboards (with insufficient voltage regulators) had more vdroop than they probably should have. This caused stability problems when overclocking, so some overclockers made modifications (called a volt mod, vdroop mod, and vdroop pencil mod) to their motherboards to reduce vdroop.
Because of this and in an effort to please overclockers, motherboard manufacturers created load line calibration to help reduce vdroop.
Load line calibration can cause your voltage to exceed what you set in the BIOS
Load line calibration goes against Intel's design and tries to keep the voltage constant at all times. However, with it turned on, we now aren't guaranteed we'll stay under the voltage that we set in the BIOS. And (as mentioned before) we're putting more stress on our motherboard's voltage regulators by trying to maintain a constant voltage.
Leave load line calibration disabled unless you really need it
If you have a decent motherboard, load-line calibration really doesn't buy you anything in terms of a higher overclock (at least it hasn't for me). It only artificially lowers the vcore that you'll have to set in your BIOS, but the CPU will still require the same amount of voltage when it's put under a load.
I'd recommend leaving load line calibration disabled unless you think that you're having a hard time achieving the overclock that you wanted and suspect excessive vdroop to be the problem.
This article is part of our ultimate overclocking guide: How to Overclock: Intel Core i7, i5, and i3 CPU Overclocking Guide.
If you're following along in the guide, you can get back to where you just were by clicking the link that follows: Overclocking the CPU.