The "path" the heat takes inside the spark plug is different for hot & cold plugs.
For a cold plug, the heat path is shorter (more direct) from the center electrode to the head. Heat gets sucked into the engine's head quickly.
For a warm plug, the heat path is longer inside the plug, which keeps the plug hotter. I had a website with a great diagram of this, but lost it- anyone know what site that was?
-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html