I broke two AC Delco plugs taking them out, where they broke clean thru the porcelain just above the main part, and I was surprised to see the electrode didn’t look like metal. Could this have been the resistor material?
Resistor plugs have been used in vehicles since the mid-sixties. The resistor reduces RFI or Radio Frequency Interference when the high energy is released when the spark plug fires.
Are you asking “what do sparkplugs that are designated “resistor type” do different than sparkplugs not so dedesignated and how do they do this”?. A “google” of “resistor type spark plugs” will give you more on this subject than I care to type.