The rear set of a o2's simply transmit a signal to the computer, it the signal is in range the computer does nothing, when u omit the cats the exhaust isn't superheated (as the purposte of cats is the heat the exhaust to burn off excess fuel) then the o2's get a different reading and flip the cel. By creating mil's you trick the computer into thinking the rear set is up to par or within the correct range at all times.