effectively restores the ignition system's efficiency.
Assumes that it needed to be restored.
You're really going to quote the NGK site?
The grounding point is still a place where few clean. In fact if that seat area is dirty, and if the you don't use thread lube, the threads can become the ground. Electricity takes whatever path it takes. Most don't clean the grounding point no matter where it is. When is the last time you wire brushed the seating area? How do you keep that thread lube off the seat?
PS: For what it's worth, I've read of some dissimilar metals type galvanic corrosion issues with some plugs and it seems to be linked to black oxided bodied plugs.
Plugs are a consumable, with a certain predicted useful life that will either not be met, or exceeded in use.
Their performance degradation is factually undeniable proven since day one of spark plug use, and still is, as only the degradation curve shape has changed over the years.
The "restoration" stated is in terms of the aforementioned degradation curve, or some form of failure, be it at the porcelain, at the electrodes, or somewhere else.
Engine sensitivity to plugs is a function of many things, a few of them being proper gap, fuel quality and type (leaded or unleaded), combustion chamber oil levels, ignition system performance, and power level.
I find their technical resources to be decent at the consumer need level and very worthy of use and referral.
Higher level info is better found elsewhere, SAE included.
Personally, I air blast the wells before loosening the plugs.
Then after plug removal I vacuum the wells (using some nominal 1/2 inch ID clear plastic hose stuck into a suitable vacuum cleaner attachment, sealed off with duct tape).
Once the plugs are out, I "read" the taper or gasket, depending on the plug.
I never see anything less than full 360 degree contact.
360 degree contact on the plug means 360 degree contact with the head.
I try not to use excessive lube.
Torquing the plug will extrude the lube from the load area.