The idea of the pair is to add o2 to the small amount of unburned fuel. Sizzle as you will it's way out the exhaust but not so perfect for complete combustion. When under vacuum there is way too much unburned fuel which creates an unplanned explosion and after the explosion you have more unburned gases (soot). Don't get me wrong this explosion burns fuel that would otherwise be exhausted (emissions) but it is not planned by Honda because of the side effects. Carbon build up is common in an engine but because the valve seats are protected they stay clean. It is logical if some sort of explosion happens it will coat surfaces with carbon and if the valve seats were open they would get the same dose as the head dome and piston surface which can have over a millimeter of carbon buildup after only a few thousand miles. You have to realize this happens over several thousand miles so you can't judge by every revolution.