On the surface, the idea is to increase the points of fuel ignition in the cylinder. Theoretically, by increasing the number of simultaneous points of ignition, the fuel/air mixture will have time to burn in a more complete manner.
Example. Take a globe about 3 feet in diameter. Fill it with a fuel air mixture. Ignite the mixture in one point of the globe. It will take X amount of time to burn to the opposite side of the globe.
Now, fill it again. Set another point opposite the original one and ignite it. The burn process will complete in approximately half the time.
That is the jest of these things. Now, they only have a chance of working in an engine where the burn cycle is not reaching 100%. In an engine turning 8,000 RPM, you might get an improvement. In an engine turning 500 RPM, it is doubtful there would be any improvement.
It is all about the burn time. Making it faster may help with some engines, and not so much with others. However, the real question is; Do plugs like these actually help decrease the burn cycle time?